William Wollaston (1766–1828), a chemist distinguished for discovering the chemical elements rhodium and palladium, also worked on methods for improving image clarity when using a magnifying glass for high magnifications. In 1814 he assembled two plano-convex lenses with a washer-like diaphragm between them and glued the assemblage together with their curved surfaces facing outward. Although the diaphragm, or stop, restricted the field of view and did little to improve the clarity of low power magnifiers, high power magnifying glasses showed definite improvement. Further experimentation by Wollaston found that the best balance between image clarity and field of view resulted when the opening through the diaphragm was one-fifth of the focal length of the combined lenses. He produced a line of magnifiers for sale and named them Periscopic lenses. Other manufacturers that reproduced Wollaston’s design have called the magnifiers Wollaston doublets.
In 1856 John Waterhouse (1806–1879), an astronomer and photographer, published a description of how to cut a slot in the side of a lens barrel holding several lenses (elements) to allow for the insertion of an opaque metal plate. Plates with openings of various diameters could be interchanged to control the width of the light’s pathway as it passed through a lens. Waterhouse found that restricting the diameter of the light cone increased the depth of the sharpness of a projected image. The slotted lens holder enabled diaphragms with openings of various sizes to adapt the lens to perform in the best way to match its application. Inserts, termed “stops” by Waterhouse, having wider openings, produced brighter images with a narrow range, or depth of focus. Stops with smaller apertures altered the performance of the lens to yield dim but sharp-in-depth images. A set of Waterhouse stops, as photographers termed them, became an invaluable tool for photographers. They remained in use until the adjustable iris made with interlocking leaves was built into photographic lenses. The settings marked on photographic lenses to indicate the diameters of the opening of an iris are still called “stops.”
David Brewster combined the optical designs of Stanhope and Wollaston into a single lens. He ground and polished convex surfaces onto the two ends of a glass cylinder creating a lens of Stanhope design. Doing so eliminated the spectral diffraction problem caused by air and glass interfaces. He then cut a groove to encircle the glass rod halfway between its two curved ends to a depth equal to one-fifth of the focal length of the lens. The encircling furrow was filled with opaque flat black paint. The restriction effectively created a Wollaston stop in a Stanhope lens. For high magnifications, Brewster’s design had the image-improving qualities of both Wollaston’s and Stanhope’s designs. Brewster’s one-piece optical construction was more durable than Wollaston’s three-piece construction, making it better for field use.
According to Allan Freer in the North British Review, Henry Coddington (1798–1845) was impressed by the image provided by Brewster’s lens during a visit. He diagrammed how the magnifying glass was made and gave the instructions to an optician he frequently worked with, a Mr. Cary, to create one for him. Reportedly, the optician believed Coddington to be the inventor of the lens design. This mistake was not unreasonable as Coddington was a professor at Trinity College, Cambridge, and the author of a respected book on optics. Cary continued making the magnifiers of the design and marketed them in his shop as Coddington lenses. Nine years later, in 1820, Brewster finally published his original work on designing the lens in the Edinburgh Journal. Coddington never laid claim to inventing the optical design, but by the time Brewster went public by publishing the design, Coddington’s name had already taken hold. The design remains popular with naturalists to this day who appreciate its small size, high power, and durability. Optical supply catalogs still offer the lenses using Coddington’s name.
The Stanhope Lens
Charles Macon (1753–1816) inherited the English title, the third Earl of Stanhope, and thereafter was publicly addressed as Lord Stanhope. He enjoyed being inventive and improving the design of useful tools. His most financially rewarding enterprise was designing and building the first cast-iron printing press in 1809. The eponymously named machine, the Stanhope press, increased the speed of turning out printed pages to 480 pages per hour — a tenfold improvement over the wooden screw press.
Another device of Stanhope’s creation was optical: a small, single-lens magnifying glass of high power. Rather than stacking several lenses together to increase magnifying power, Stanhope began with a solid glass rod and ground its two ends into convex lenses. He did this in 1801 before achromatic lens designs were able to control the color fringing that highly magnified images displayed. The fringing problem, termed chromatic aberration, was a limiting factor in designing optics that could yield high magnification. Stanhope’s glass cylinder design eliminated the glass-to-air-to-glass interface between two lenses. The bending of light as it passed through media of different densities created the spectral aura that limited resolution. Stanhope’s higher power lens design corrected some of the aberrations inherent in multiple lens design but had the disadvantage of becoming increasingly smaller in diameter with increasing magnifications. A high power Stanhope lens could not be made larger than the size of a pea.
Forty years after Stanhope’s death, the French entrepreneur René Dragon was looking for more practical applications of John Dancer’s microphotography process than making novelty microscope slides. Experimenting with Stanhope lenses, Dragon realized that a solid glass cylinder with one end ground to a particular curvature could have its focal point fall within the glass rod. If the end of the glass cylinder was cut and polished into a flat face at exactly that distance, then anything pressed against the plane surface would be in sharp focus when viewed from the convex side. Dragon realized that gluing a microphotograph to the plane surface of such a lens would provide a way to see microphotographs without using a microscope. He invented the novelty peep viewer, but it still carried Stanhope’s name.
A problem with a lens of Stanhope’s design is that the lens’ diameter shrinks as its magnifying power increases. Without a custom-made holder, a high power Stanhope lens is physically challenging to bring up to one’s eye to look through. Dragon, as did Dancer, realized that the novelty market would be the best application of the device. Low-cost souvenirs and jewelry would be the most accessible holders of Stanhope lenses to design with a wide range of possible markets. Nineteenth-century jewelry containing Stanhope lenses, particularly those made in France, are desirable antiques among contemporary collectors.
According to Carpenter’s book on microscopy, Stanhope lenses can be made to magnify 100 to 150 diameters. This magnification is sufficient for viewing diatoms if a Stanhope lens’s flat surface is dipped into a drop of water containing the organisms. Carpenter recommends that Stanhope lenses are inexpensive and can be purchased at toy dealers. (Carpenter, 1881)
In the July 1866 issue of Hardwick’s Science Gossip, T. P. Barkas (1819 – 1891) authored a letter about seaside diatom collecting in which he suggested a solution for the difficulty of carrying a microscope about the field. He recommended using a Stanhope lens for examining water samples while searching for diatoms at the seashore. Barkas used a lens he obtained from a toy dealer and, by his estimate, was capable of magnifying 100x. He offered to mail similar lenses to readers, his gratis, for the price of a postage stamp. Barkas called the lenses “Stanhopescopes.” He recommended gluing the tiny lens over a small hole made in the cap of a small collecting container for handling. The drop of water being searched for diatoms should be placed on the flat side of the lens and looked through from its curved side while being held toward a light source.
John Bockett, the inventor of the eponymous microscope lamp, answered Barkas’ letter in the September issue of Hardwick’s of the same year. Bockett stated that he obtained one of Barkas’ Stanhopes and believed the image quality of a Coddington lens was superior to that obtained by using a Stanhopescope. He did claim to have excellent results when he further modified the procedure by using the two lenses together. Bockett recommended using the Stanhopescope as a condensing lens. His method was to place a drop of water onto the flat surface of the Stanhope but illuminated the sample through the curved side of the lens. Bockett then examined the water sample on the flat side of the Stanhope through a Coddington lens. He ended by writing, “If this hint is worth anything or not, I will leave to others.”
Barkas replied in the October letters section to what he described as Bockett’s “Somewhat severe criticism of the Stanhopescope.” He pled that Bockett unfairly failed to consider how inexpensively Stanhope lenses were produced, and, that being the case, the great variance in quality there must be in such cheaply made lenses.
Barkas was born into a family that owned and operated a construction business. After attaining a sufficient degree of financial independence, he left the family business turning to a public lecturing about his passion, spiritualism. Because of his work in microscopy, he proclaimed himself a scientist and, as the headline in the weekly London newsletter The Medium and Daybreak, December 18. 1885, declared spiritualism was attested to by science. Barkas became well known for his public séances, where mediums would communicate with the spiritual world to answer questions from the audience about science. Understandably, such methodology was frowned upon by others in the sciences.