16 Transactions of the Society. 



not suffer from the above-mentioned defects. Four typical 

 members of the series have been designed and two actually 

 •constructed. Tests of these have shown that the elimination of 

 the difficulties' above mentioned has been effected with complete 

 success. The construction of these eye-pieces may be briefly 

 described as follows :— In all powers the top combination is fully 

 achromatized, whether it be a simple doublet or a more complicated 

 system. Similarly in all powers the compensation is provided by 

 a combination of crown and flint glass lenses, which are of equal 

 and opposite power, so that the whole has no focus for rays in the 

 middle of the spectrum. Eays of short wave length are, however, 

 laent towards the axis, while rays of long wave length are bent 

 ■away from it. This causes images produced by violet and blue 

 rays to be decreased in size, and images produced by red and orange 

 rays to be increased, relative to images formed by yellow and 

 green rays. 



Suppose the a-focal combination to correspond with the plane 

 •of the virtual image formed by the objective, then it is clear that 

 no change in the relative size of the images produced by different 

 ■coloured rays has been produced, and the eye-piece is therefore 

 uncompensated. As the a-focal combination is placed further 

 from the image plane, so the relative size of the images is changed. 

 Thus, if it be placed between the image and the eye, then violet 

 images are magnified and red images are diminished (and the eye- 

 piece as a whole becomes more uncompensated) ; if, on the other 

 hand, the combination be placed below the image (in between the 

 'Objective and its image), then the effect on the images is reversed 

 and therefore compensation is introduced, the amount of which 

 varies with the distance between the virtual image and the 

 ■combination. 



If in the a-focal combination the negative lens is of flint glass 

 and the positive of crown, then the effects are the reverse of those 

 <lescribed above. The series of eye-pieces made according to the 

 above construction will now be briefly described ; further particulars 

 are shown in the diagram. 



X 4 Eijc-piecc. — This searcher eye-piece is of Huygenian type. 

 It consists of achromatic top lens and bi-convex flint glass field 

 lens ; the whole forming an over-corrected compensating system. 

 Between the field lens and the diaphragm is placed the a-focal 

 combination, which consists of negative fliit glass lens and positive 

 crown lens of equal power. 



When this a-focal doublet is close to the diaphragm the over- 

 correction produced by the flint glass field lens takes effect ; 

 when the doublet is slid down close to the field lens, however, 

 its over-correction is neutralized and the eye-piece rendered un- 

 compensating. A " watch glass " is fitted to the top of this eye- 

 piece in order to exclude dust. 



