250 COUNT CASTRACANE, ON ILLUMINATION. 
photographic art, I have for several years endeavoured to find 
a useful employment for my leisure hours in the application 
of photography to the reproduction of the wonders of nature 
as disclosed in the microscope. With this object, I have pro- 
cured a very perfect microscope, constructed by the late Pro- 
fessor Amici. But I soon perceived that in order to employ 
the microscope at all usefully demanded a full knowledge of 
the instrument and of the use of each of its parts, and espe- 
cially as to the means of modifying the direction and intensity 
of the light. In order to overcome the difficulty arising from 
the want of this full knowledge of the instrument, I availed 
myself of the assistance of the works of Dujardin and of 
Quekett ; but I was still more assisted in this regard by some 
practical lessons which I had the good fortune to receive from 
Professor Amici. Amongst various other methods of pro- 
ceeding, the professor showed me, and induced me to try, a 
mode of illumination discovered by himself for general use 
with the microscope, but which had not been at that time 
made publicly known. The method in question consisted in 
the employment of one of the component rays of the solar 
spectrum, which was made to fall on the mirror of the micro- 
scope, or on his lenticular prism. I was enabled to make 
large use of this kind of monochromatic light with great con- 
venience after I had obtained a large heliostat, furnished with 
a mirror of thirty by fifteen centimétres, newly constructed 
by Dubosc, of Paris, on the plan and under the direction of 
M. Foucault. With the aid of this instrument, placed at a 
short distance from the microscope, I could obtain a field 
wholly illuminated by homogeneous coicured light. This 
kind of light, bemg incapable of further decomposition, ap- 
peared to me calculated to obviate any defect in the achro- 
matism, either in the objective or ocular, as well as to get rid 
of those fringes caused by interference which are formed es- 
pecially round the margins of objects. As regards the esti- 
mation of the power of the various objectives belonging to 
my microscope, it may be remarked that, whilst with white 
light I was able to see the structural characters of Pleuro- 
sigma angulatum as a series of points with the fifth and 
stronger objective, and with the fourth simply as lines, with 
the monochromatic light the points were exhibited with the 
utmost distinctness, not only with the fifth and quarter and 
with the lowest eye-piece, but could be seen with equal dis- 
tinctness with the third, which has much less magnifying and 
penetrating power, and which, with ordinary white light, af- 
forded a somewhat cloudy image. At present I have been 
unable to determine exactly the different degree of penetra- 
