10 DR. WOLLASTON’S DESCRIPTION OF A MICROSCOPIC DOUBLET. 
The cost, however, of such a lens in comparison with glass, as well as the 
readiness with which any number and variety of the latter kind can be pro- 
cured, led me to consider what simple combinations of them might perhaps 
equal the sapphire lens in performance, without great cost, or difficulty of con- 
struction ; and though both Mr. Herschel and Professor Airy have recently 
applied their superior talents to the analytical investigation of this subject, it 
seemed not impossible that the more humble efforts of a mere experimentalist, 
might be rewarded by some useful results. 
The consideration of that form of eye-piece for astronomical telescopes 
called Huygenian, suggested the probability that a similar combination should 
have a similar advantage, of correcting both chromatic and spherical aber- 
ration, if employed in an opposite direction as a microscope. 
The construction which I found convenient in my trials, may be not unaptly 
compared to two thimbles fitted one within the other by screwing, and each 
perforated at the extremity. By this construction, two suitable plano-convex 
lenses fixed in these perforations, may, because of their plane surfaces, have 
their axes easily placed in the same line ; and their distance from each other 
may be so varied, by screwing, as to produce the best effect of which they are 
susceptible. 
As far as my trials have hitherto gone, I am led to consider the proportion 
of 3 to 1 as nearly the best for the relation of the foci of these lenses ; and 
their joint performance to be the most perfect, when the distance between 
their plane surfaces is about l x 4 o of the shorter focus. But as all the lenses 
I possess are not similar segments of spheres, or of the same relative thickness, 
I could not expect exact uniformity in the results. 
The following is a description of the apparatus which I have employed. 
T, U, B, E, (Plate II. fig. 1.) represents a tube about six inches long, and of 
such a diameter as to preclude any reflexion of false light from its sides ; and 
the better to insure this, the inside of the tube should be blackened. At the 
top of the tube, or within it, at a small distance from the top, is placed either 
a plano-convex lens E, T, or one properly crossed, so as to have the least aber- 
ration, about three-quarters of an inch focus, having its plane side next the 
object to be viewed ; and at the bottom is a circular perforation A, of about 
three-tenths of an inch diameter, for limiting the light reflected from the plane 
