10 DR. WOLLASTON'S DESCRIPTION OP 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 1^% 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 



