194 OPTICAL PROJECTION 



or eye-piece. Leitz's new formulae give fair results. But 

 the best dry lenses I have found at any moderate price are 

 Herr Eeichert's new series (termed by Mr. E. M. Nelson semi- 

 apochromatics) No. 8 (f) 20s., No. 6 (J-) 80s., and No. .7 A (|) 

 86s., all which work beautifully by either of the three methods. 

 The last is a truly wonderful lens at its price, and both it 

 and the J are far the best projection lenses I have been able 

 to find of these powers. I may add here, that some really 

 good lenses, when used with brilliant lights such as projection 

 demands, give a ' mist ' over the image purely from flare, or 

 reflection in the lens mount, and which is removed by care- 

 ful blackening. I have drawn the attention of Herr Keichert 

 and others specially to this, and am sure the point has not 

 yet received the attention which it demands, now lenses are so 

 often used for photographic purposes with these strong lights. 



With the electric arc, Zeiss's apochromatics give fine results 

 over the small field of his own projection eye-pieces, but only 

 so used. They cannot be used with satisfaction direct, or with 

 an amplifier. 1 The same in both respects may be said of the 

 wide-angled dry lenses of Messrs. Beck. Few of these lenses, 

 however, have the ready adaptability of the Eeichert series 

 for projection work. Messrs. Powell and Lealand's apochro- 

 matics are much better in this respect, and their j inch of 

 0*95 NA, for its combination of exquisite definition with a very 

 large flat field, appears to me at present the most ideally 

 perfect lens in the world. Even at its high price of 111., it is 

 well worth purchasing for constant histological projection 

 work, of which (with eye-pieces) it will cover a great range. 



Of immersion lenses, the best for projection at any mode- 



1 I have been greatly disappointed in these lenses for projection. They 

 only exhibit a very small bit of the centre of their field in focus, and appear 

 to be constructed chiefly with a view to photo-micrography. This seems to 

 me a great practical mistake, and I moreover believe that certain conclusions 

 of Prof. Abbe relating to the nature of the image of minute structure, some of 

 which have been demonstrated to be erroneous by Mr. E. M. Nelson (Quekett 

 Club Jo., July 1890), are chiefly due to this character of his own lenses, and 

 the fact that different zones of the image are in such widely different focal 

 planes. 



