]0 APOCHROMATIC OBJECTIVES. 



The results given by Powell and Lealand's i/io in. apo- 

 chromatic eye-piece, x 20, and their 1/20 in. achromatic 

 X 10 were almost indistinguishable. There was a little less 

 colour with the i/io in., but the 1/20 in. gave, if anything, 

 sharper definition. 



Powell and Lealand's 1/25 achromatic of N.A. i'38 and eye- 

 pieced X 8 gave as little colour as the i/io in. apochromatic 

 X 20, but the definition was not quite so crisp, doubtless on 

 account of its smaller aperture. The same might be said of the 

 1/12 in. of N.A. i'43 x 18. 



Zeiss's water immersion apochromatic of N.A. i"25 and 2-5 

 millimetres focal length, and Powell and Lealand's new formula 

 1/8 in. water immersion achromatic were then tested with these 

 eye-pieces in the same way. The corrections for spherical aber- 

 rations with both these glasses were equally good, but the apo- 

 chromatic by Zeiss gave an image almost entirely free from colour 

 even with the highest-power eye-pieces. 



Zeiss's dry apochromatic of "95 aperture and 6 mm. focal 

 length gave an absolutely water-white image even when used 

 X 20, and was otherwise superior to 1/4 in. of loo'' by 

 Swift and Son, with which it was compared, but it did not give 

 so flat a field of view. The 1/4 in. of loo*^ by Swift is the 

 best 1/4 in. of the ordinary achromatic type the writer has 

 seen ; the Zeiss dry apochromatic had, however, the advantage in 

 point of aperture. 



The results obtained with Powell and Lealand's i/io in. 

 apochromatic and their 1/20 achromatic, and also with Zeiss's, 

 and Powell and Lealand's water immersions seemed rather to 

 lean towards the view that by eliminating the secondary spectrum 

 some of the resolving power is lost. On the other hand, the 

 apochromatics are more pleasant to work with and better adapted 

 for photo-micrography, though a projection eye-piece does much 

 in this respect for the ordinary glasses. 



On the whole, the advantage to be derived from apochromatic 

 lenses seems smaller than has been anticipated. As has already 

 been noticed by Dr. Dallinger, the compensating eye-pieces are 

 perhaps the more important factor in the advance made. 



Ordinary achromatics have been brought to such a state of 



