822 Transactions of the Society. 



The leDses having been ground and set in the brass work with 

 all possible care, according to the formula, the two made at the 

 workshop of Mr. Zeiss were perfectly corrected by a very slight 

 alteration (some hundredths of a millimetre) of the distance 

 between the second and the third lens of the front part. The 

 apertures of both objectives, observed with the light of a soda 

 flame, was found 1'40 to 1'41 by means of an apertometer with 

 flint-glass disk and the same by an independent micrometric method. 

 Notwithstanding the various delicate and novel manipulations 

 which were necessary for exact polishing and setting of a front 

 lens of this peculiar kind, the performance of the two objectives 

 was perfectly satisfactory. The resolving power gave evidence of 

 the increased aperture. Ordinary specimens of Amphiiyleura i^el- 

 lucicla are rather coarse test-objects with an aperture of this amount, 

 as they are resolvable by a moderately oblique pencil. There is 

 no doubt that on balsam-mounted preparations, striations up to 

 5000 lines per millimetre (125,000 per inch) would be readily 

 resolved by an incident pencil of the utmost obliquity, and still 

 finer photographically. The difference of chromatic correction 

 between the central and the marginal zone of the aperture is in 

 fact imperceptible ; the middle of the field yields nearly white 

 images, with central light, and secondary colours only, with light 

 of extreme obliquity. The definition with the deeper eye-pieces 

 proves decidedly superior to the objectives of similar focal length, 

 made for homogeneous immersion on the ordinary plan of correc- 

 tion ; and as the other circumstances connected with this con- 

 struction are somewhat less favourable, there can be no doubt that 

 the improvement is due solely to the removal of the residuary 

 spherical aberration. 



As was anticipated from the formula, the chromatic difierence 

 of amplification appeared much more perceptible than with other 

 objectives. I added therefore a correcting lens below the eye- 

 piece — a convex flint- and a concave crown-lens cemented to- 

 gether, calculated for neutralizing one another in respect to the 

 middle rays of the spectrum, but with exceeding chromatism of the 

 flint. This lens therefore performs like a plane parallel i^late for 

 the middle rays, as a collective lens for the blue, and as a dis- 

 persive lens for the red rays. Inserted at the end of the tube by a 

 suitable adapter at a short distance from the field-glass of the 

 ocular, it introduces— owing to its position — no perceptible aberra- 

 tion, neither chromatic nor spherical ; but it corrects the path of 

 the coloured pencils outside the axis by continuously increasing 

 prismatic deflections. This device performs quite satisfactorily 

 with all eye-pieces, except very low ones. 



For photographic performance an objective of this construction 



