3f84 Dr Wollaston's Microscopical Doublet. 



more humble efforts of a mere experimentalist, might be re- 

 warded by some useful results. 



The oonsideration of that form of eye-piece for astronomi- 

 cal telescopes called Huygenian, suggested the probability that 

 a similar combination should have a similar advantage, of cor- 

 recting both chromatic and spherical aberration, 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 with- 

 in the other by screwing, and each perforated at the extremi- 

 ty. By this construction, two suitable plano-convex lenses 

 fixed in these perforations, may, because of their plane sur- 

 faces, 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 consi- 

 der 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 sur- 

 faces is about l/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 n0t expect exact uniformity in the 

 results. 



The following is a description of the apparatus which I 

 have employed. 



T, U, B, E, (Plate III. Fig. 7,) represents a tube about six 

 inches long, and of such a diameter as to preclude any reflec- 

 tion 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 cros- 

 sed, so as to have the least aberration, about three-quarters of 

 an inch focus, having its plane side next the object to be view- 

 ed ; 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 mirror R, and which is to be brought to a focus at«, 

 giving a neat image of the perforation A at the distance of about 

 eight-tenths of an inch from the lens E T, and in the same plane 

 as the object which is to be examined. The length of the tube 



