COUNT CASTRACANE, ON ILLUMINATION. 251 
tion obtainable by the use of the different coloured rays ; but 
think it may be stated that the best optical results are derived 
from the green and turquoise-blue. I have said the optical 
results, because, with respect to the chemical or actinic effects, 
which will present a greater interest to me, I am unable at 
present to say anything. In the mean time, however, I am 
endeavouring to adapt the monochromatic illumimation to 
the apparatus which I employ for the obtainmg of photo- 
graphic representations of microscopic objects ; and I regret 
this delay the more because I believe the best result may be 
expected from the adoption of this mode of illumination, 
which opinion has decided my choice of the mode best adapted 
for the execution of a work which I flatter myself may be of 
some utility in the study of a branch of natural history. 
The work I have proposed to myself, and towards which I 
have already made some trials, is an atlas of living and fossil 
diatoms, native and foreign, and as complete as I may be 
enabled to render it, trusting in the aid I may hope to obtain 
from those who have made these organisms a special subject 
of study; and at the same time neglecting no opportunity (as 
I have already done) of adding to a beautiful small collection 
which I have procured from MM. Bourgogne, of Paris, and 
Smith and Beck, of London. But before entering upon such 
an undertaking, I was desirous of ascertaining whether it 
would be possible to reproduce some of the minute particu- 
lars which are presented in the siliceous skeleton of these 
curious organisms, amongst which are some which exhibit 
such extreme minuteness of detail that, not being capable of 
being resolved except by the better class of instruments, they 
have been selected by microscopists as tests of the penetration 
and definition of the better and more powerful objectives. 
And as among these Plewrosigma angulatum is so difficult to 
be made out in its most minute structural peculiarities, 
“which,” as observed by Griffith and Renfrey in the ‘ Micro- 
scopical Dictionary,’ ‘are invisible by direct light, however 
large or small may be the aperture of the object-glass, or 
however perfect its definition” (although this does not accord 
with the fact that my fifth objective shows them to perfection 
with direct and central illumination)* I have, consequently, 
tried to obtain a photographic figure of this diatom, and have 
procured it with the utmost possible distinctness and sharp- 
* Although the case may have been as stated at the time the ‘ Micro- 
graphic Dictionary’ was printed, it is, as is well known, widely different 
now, when we have abundance of object-glasses fully capable of defining the 
nnEings of P. angulatum, and on still more difficult tests, with direct 
ight, 
