074 Transactions of the Society. 



superiority in a general way of glycerine as the immersion fluid 

 over water, or of oil of cedarwood and other liquids closely approxi- 

 mating crown glass in refraction and dispersion over glycerine. 

 But this superiority does not occur merely because increased angle 

 is thus rendered possible. In fact, as the angle of total reflexion from 

 crown glass to water is rather more than 60°, it is by no means 

 theoretically impossible to construct water-immersion objectives 

 with angle as great as the oil-immersion objectives of Zeiss, or the 

 glycerine-immersion objectives of Spencer. The difficulty in this 

 case is to correct the aberrations which are inevitably produced by 

 refraction at the upper surface of the thin glass cover and the flat 

 surface of the objective front, l^hese aberrations are entirely absent 

 when the immersion fluid has the same refraction and dispersion as 

 the glass on each side of it ; are comparatively slight in the case of 

 glycerine ; much more considerable with water, and greatest in the 

 case of the dry objectives. Professor Abbe, in the paper already 

 cited, has drawn attention to this circumstance, which appears to me 

 even more important than the fact that with homogeneous immer- 

 sion there is no loss of light by reflexion at the front surface of the 

 objective, and with glycerine immersion very little ; but this also 

 must have its influence. 



Taking all the circumstances into consideration, I am disposed 

 to expect further improvement in objectives in the direction of homo- 

 geneous immersion rather than glycerine immersion. In the case of 

 homogeneous immersion, too, we have the great advantage of being 

 able to dispense with the screw collar for cover correction, and all 

 the deplorable loss of time entailed by the use of that contrivance, 

 which is absolutely required in the case of glycerine and water- 

 immersion objectives. 



For this reason, in my ordinary work I give my Zeiss ^ the 

 preference over the objectives I have named as somewhat sur- 

 passing it in defining power ; because it gives instantly results that 

 are not far inferior to the best I can obtain from the others with 

 much pains and loss of time. 



Finally, to illustrate the superb performance of the Zeiss xV on 

 dry Am]ylii])Jeura, I have added to the series a photograph (No. 14) 

 of a very delicate frustule on a slide of Ampliipleura pellucida 

 from the Bridge of Allan, Scotland, mounted by my friend Pro- 

 fessor Hamilton L. Smith, of Geneva, New York. This frustule is 

 only 29 ten-thousandths of an inch long, and has 105 striae to the 

 thousandth of an inch. It is magnified 3400 diameters. 



