436 



Dr. Royston-Pigott on 



[June 19, 



an objective, that a dry objective might in many cases be transformed 

 into an "immersion" simply by advancing the eyepiece; also that a 

 sufficient variation of interval between the front lenses might in many 

 cases enable a dry lens to act as an immersion. 



[The immersion principle is valuable for the increased volume of the 

 cone of rays radiant from the illuminated particle mounted in balsam, 

 a much larger pencil reaching the objective via water than can possibly be 

 effected via air, the " critical angles " of total internal reflection which 

 determine the form of the caustic being so much larger in passing from 

 glass into water than into air. I have shown elsewhere that the volume 

 of the cone of rays transmitted from a radiant particle placed in balsam 

 and surmounted with a thin glass cover is about four times greater via 

 water than via air ; that result is explanatory of the greater brightness 

 of the immersion lens.] 



The question of the spherical and chromatic aberrations of eyepieces 

 has occupied the attention of the most distinguished mathematicians, and 

 may theoretically be considered nearly exhausted; yet the practical 

 detection of its existence is known to few, as it is liable to be mixed up 

 with the objective aberration. The methods described are equally 

 applicable to eyepieces as to objectives. (Appendix B.) 



The construction of a compound compensating eyepiece which should 

 be almost perfectly free from this residuum next engaged my attention. 



Prom the discovery that the advance of an eyepiece towards the ob- 

 jective caused a violent under correction in the refocused objective, it became 

 evident that a shortened microscope could be employed as a compound 

 eyepiece nearly free from the usual aberrations, provided its object-glass 

 were properly overcorrected, as compared with its performance at the usual 

 standard distance of ten inches. 



The new eyepiece is finally corrected on the circular solar spectrum 

 (herein described), being regarded and treated as a real microscope. Its 

 object-glass, considering the exceedingly small pencil engaging it, may 

 conveniently be formed of slightly overcorrected achromatic lenses, com- 

 pensated by a variable interval. I have found an inch focal length suffi- 

 ciently deep, mounted with a low eyepiece. The substitution of this 

 compensating eyepiece for the ordinary deep Huyghenians afforded that 

 degree of comfort in observation corresponding to enlarged pencils. 



After adjustment it is quite as applicable to examine the performance 

 of telescopes as microscopes. The adjustment is thus accomplished : — 



1. The instrument, mounted as a complete microscope, was adjusted 

 for the most perfect definition on an uncovered object ; and supposing 

 the glasses A, B (adjustable by a variable interval) defined perfectly with 

 the usual length of tube 10 inches, they require overcorrection* or 

 separation for a shorter tube of 6 inches. 



* Considering the small angular aperture, a single set of achromatic lenses might be 



