41 



On the Finer Structure of Certain Diatoms. 



Part II. 



By E. M. Nelson and G. C Karop. 



Plate IV. 



(Taken as read January 28th, 1SS7.J 



Since our last paper on this subject (read March 26th, 1S86) an 

 achromatic oil immersion condenser has been made by Mr. T. Powell, 

 Mr. Nelson having, in 1882, suggested to him the necessity for 

 achromatising the then chromatic oil condenser. This has enabled 

 us to illuminate objectives by solid axial cones of larger angle 

 than before ; the spherical aberration of a chromatic condenser 

 being so great that only the rays passing through the centre or 

 thronsrh a narrow zone of the condenser could be focussed on the 

 object at one time. The result has been a marked increase in 

 resolution. In illustration of this increased resolution we would 

 refer you to Fig. 6, which is a drawing of an areolation of the same 

 valve of Tsthmia nervosa, which we figured in our former paper 

 (Plate 17, Fig. 2. Series II, Vol. ii). The straight bars of silex 

 by which the central delicate perforated membrane was shown to 

 be attached to the margin of the areolation now have a trabecular 

 appearance ; the delicate membrane extends to the edge of the 

 large areolation, and has perforations more difficult to resolve 

 than those in the centre. We wish to point out that this is not a 

 correction of misinterpretation of optical images, but a clear case of 

 increased resolution, due to an improvement in optical appliances. 

 Even now we do not wish to lay any claim to finality, but to show 

 that every advance in perfecting instrumental appliances is at- 

 tended by an increased gain in our knowledge of structure. In 

 addition to the new condenser, we have used Professor Abbe's new 

 compensating eye-pieces, which give sharper images than those of 

 the Huyghenian construction. Before proceeding further we wish 

 to record our great obligation to Mr. G. Sturt for the very kind 

 Way in which he has placed his magnificent collection of diatoms at 



