294 
Transactions of the 
C Monthly Microscopical 
Journal, Dec. 1, 1869. 
in the Nobert's plate as distinctly as they are shown in the photo- 
graphs submitted, I must presume simply that he has not illuminated 
the object with monochromatic hght as directed in my paper. 
Although certainly it must be admitted that the Nobert's plate is 
one of those objects in which the photograph most nearly approaches 
the beauty and detail of the original, and it must be of course 
apparent that a photograph will frequently contain details which in 
the microscope have escaped the observation of feeble or inattentive 
eyes. 
In conclusion, I must say a few words about the photographs 
taken by Dr. Curtis, copies of which accompany this paper. 
The original negatives were taken with the immersion yVth, 
without an eye-piece, the distance of the sensitive-plate being such 
as to give as nearly as possible a thousand diameters. On these 
negatives, or on glass positives printed from them, the count of the 
lines may be made under a low magnifying power. Paper prints 
taken directly from the original negatives are very unsatisfactory, 
the texture of the paper interfering with the printing of such fine 
lines. On the other hand, enlarged prints lose so much detail that 
the difficulty of distinguishing the last real line in any band from 
the spectral lines on its margin, is so much increased as to make a 
satisfactory count impossible. I therefore send with this paper 
two glass positives ; the first of which may be used for the study 
of the sixteenth, seventeenth, and eighteenth bands, while the 
second is especially intended for the nineteenth. I also send three 
paper prints enlarged to two thousand diameters, which will serve 
to show the general appearance of the lines, but which cannot be 
rehed upon to guide in the count for the reasons just stated. 
