ZOOLOGY A.ND BOTANY, MICROSCOPY, ETC. 1071 



powers. Dr. J. D. Cox's experience does not accord with this state- 

 ment, and he makes the following remarks on the subject : — 



" K the statement had been that a sharp picture may be taken 

 when the object is exactly in focus with a high power I should not 

 take exception to it, and I incline to think that this is what has been 

 meant. But a sharp picture may be either a positive or a negative 

 of the visual image seen in the Microscope, and in my own work so 

 many examples have turned out to be positives when I expected them 

 to be negatives, that I have been led to make an investigation of the 

 subject, in which the evidence tends strongly to show that with our 

 best high-power lenses the image fixed upon the sensitive plate is a 

 positive instead of being a negative, and consequently the paper 

 prints from this are negatives and not positives. 



It would be very easy to overlook this difference in a large class 

 of photo-micrographs, because, in an alternation of dark and light 

 lines, or dark and light spaces, it often matters little which of a pair 

 is light or dark ; the picture may be equally clear and satisfactory 

 either way. In the case of a large majority of the microscopic objects 

 photographed, either the positive 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 until, in efibrts 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 areola?, this often amounted 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 objects 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 central illumination, 

 even after our best efforts to do so. For example, jilate No. 23 of my 

 broken shell series was thus taken with liglit 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 represents, appear as deep cups, 

 whilst, if it be turned round so as to change places of top and bottom, 

 they appear as projecting bosses. 



No. .01 of tlic same series was the first in which I distinctly 

 marked in my note-book the fact that the dots in that diatom, 

 Maatofjloia angulata, appeared dark in the instrument, but light in 

 the photograph print. The difference of effect was least imjjortant 

 in shells which have an even, smooth film of comparatively little 

 thickness, and the greatest in those in which the diatom seems to have 

 strongly marked bars separating the lines of areolae, as in Pleuroaigma 

 balticum. 



In a number of cases in which the plates were originally tajicn 



