18 Transactions, of the Society. 



X 8 Eye-piece. — This medium-power lens is intended to carry 

 a ruled glass micrometer, and is of the Kelner type. It consists of 

 achromatic top lens and plano-convex crown glass field lens, to the 

 plane surface of which is cemented the micrometer. At the lower 

 end of the eye-piece tube is a low-power crown glass lens which 

 renders the eye-piece parfocal with the others of the series, and 

 slightly under-corrected. Between this and the field lens slides the 

 over-corrected a-focal combination. This when close to the field 

 lens neutralizes the under-correction introduced by the bottom 

 lens, thus rendering the eye-piece neutral. On moving the 

 a-focal doublet downward a variable amount of over-correction is 

 introduced. 



X 16 Eye-piece. — This high-power eye-piece is of the Eamsden 

 type, and consists of an achromatic top combination, formed l^y 

 an over-compensated triplet and uncorrected field lens. The 

 a-focal combination is mounted so as to be adjustable at varying 

 distances below the diaphragm, thus introducing the required 

 amount of compensation. 



X 32 Eye-piece. — This eye-piece is intended for objective test- 

 ing, and like the last is of Eamsden type. The positions occupied 

 by the corrected and uncorrected lenses is reversed, however, so as 

 to place the Eamsden disc as far as possible from the top of the 

 eye-piece. 



A short distance below the diaphragm is a crown glass plano- 

 concave lens, the functions of which are — 



1. To render this eye-piece parfocal with the others in the series. 



2. To magnify the image (in this respect it acts like a tele- 

 photo photographic lens). 



This lens was found to have two additional beneficial effects : 

 it increased the distance between the Eamsden disc and the top 

 of the eye-piece, and it rendered the eye-piece initially under- 

 compensated. 



Below this negative lens is mounted the a-focal combination, 

 which functions as before. 



In order to avoid a thick correcting lens with deep curves, it 

 would be an advantage to construct the top lens of some material 

 of low dispersive power, such as quartz, thus reducing the chromatic 

 under-correction which is necessarily introduced. 



Of these eye-pieces x 4 and x 16 have been constructed and 

 tested. In both the definition was found to be good and the field 

 flat. Compensation could be easily adjusted, the scope being 

 sufficient for the ordinary range of objectives (including Zeiss 

 apochromatics, except with the X 16 eye-piece, in which the a-focal 

 combination had not been given sufficient power). The variation 

 in compensation was not accompanied by any change in focus, 

 magnification or equivalent tube length. Both eye-pieces were 

 parfocal with others belonging to the author. 



