194 



THE AMERICAN MONTHLY 



[October, 



tive or negative image would be good 

 enough for the purpose intended : so 

 good that a close examination of the 

 point I am now suggesting would 

 hardly occur to one. This, in fact, 

 was my own experience vmtil, in 

 efforts to get a good picture of the 

 broken edge of fragments of the finer 

 diatoms, my attention was arrested 

 by the fact that the appearances seen 

 by the eye were often reversed in the 

 print from the supposed negative 

 which I had taken. As, in dealing 

 with minute areolae this often amount- 

 ed to showing a projection where I 

 had seen an apparent depression, and 

 vice versa ^ it became in effect a 

 failure to photograph what I had 

 seen, and challenged my best efforts 

 to overcome the difficulty. If the 

 illumination of such transparent ob- 

 jects as diatoms were always by a 

 perfectly central beam of parallel rays 

 of light, there would be no practical 

 difference whether they showed light 

 upon a dark ground, or the reverse. 

 But we rarely get such exactly cen- 

 tral illumination, even after our best 

 efforts to do so. For example, plate 

 No. 23 of my broken shell series was 

 thus taken with light intended to be 

 strictly central, a diaphragm being 

 behind the achromatic condenser, 

 which had a small circular hole in it, 

 limiting the illuminating rays to the 

 small central portion of the condenser. 

 Yet in one position the central areolae 

 of the Coscinodiscus which it repre- 

 sents, appear as deep cups, whilst, if 

 it be turned around so as to change 

 places of top and bottom, they appear 

 as projecting bosses. 



No. 51 of the same^ series was the 

 first in which I distinctly marked in 

 my note-book the fact that the dots in 

 that diatom, Mastogloia angtdata^ 

 appeared dark in the instrument, but 

 light in the photograph print. The 

 difference of effect was least impor- 

 tant in shells which are an even, 

 smooth film of comparatively little 

 thickness, and greatest in those in 

 which the diatom seems to have 

 strongly marked bars separating the 



lines of areolae, as in Pleurosigma 

 Balticum. 



In a number of cases in which the 

 plates were originally taken with a 

 sharp focus upon the view of the 

 shell which I desired, I have taken 

 transparencies from them by contact, 

 and using these last as negatives from ' 

 which to print the paper prints, 

 I have found that these last are, ac- 

 cording to my notes, what the former 

 should have been if there were no 

 difference between the visual and the 

 actinic focus. A few Of these have 

 been prepared for exhibition to the 

 Societ}^ The prints taken from the 

 second plates are marked ' positives ' 

 of the originals, and are in fact the 

 true representation of the object as I 

 saw it when taking the original photo- 

 graph. They are, No. 66, Navicula 

 serians^ Kutz., taken with a Spencer 

 yL- objective, balsam angle 125°, with 

 No. 118 as the positive from it. 



No. 60, Pleurosigma Forniosum^ 

 W. Sm., taken with a Spencer ^^ 

 objective, balsam angle 108°, with 

 No. 122 as the positive from it. 



No. 83, Pleurosigma Formosum^ 

 W. Sm., taken with a Whales y^^ ob- 

 jective, balsam angle 82°, with No. 

 119 as the positive from it. 



No. no, Pleurosigma Balticum^ 

 W. Sm., taken with a Zeiss ^^ ob- 

 jective, balsam angle 116°, with No. 

 113 as the positive from it. 



The objectives are all of the first 

 class, and it is safe to assume that 

 what holds true with them will be 

 found true with any of our best 

 glasses. 



In taking the original photographs, 

 I used a plain plate of glass instead of 

 the usual gi'ound glass screen in the 

 camera, and focussed by the aid of a 

 Dorlot focussing glass. 



The examples to which I have refer- 

 red would seem to warrant the conclu- 

 sion that in using high-power objec- 

 tives the difference between the visual 

 and the actinic focus is the equivalent 

 of that between a positive and negative 

 image of the object, when the details 

 have passed a certain limit in fine- 



