254 WOODWARD, ON MONOCHROMATIC ILLUMINATION. 
by the ordinary achromatic condenser, and the objects viewed 
preferably by objectives specially constructed for the violet 
ray, such as have been made for the branch of the Army 
Medical Museum at Washington, under my charge, by Mr. 
Wm. Wales, of Fort Lee, New Jersey. I find, however, 
that for ordinary achromatic objectives of high power, such 
as the J.th, =th, and th of Messrs. Powell and Lealand, 
of London, or the No. 11 immersion lens of Mons. E. 
Hartnack, of Paris, the special correction may be dispensed 
with, and good results obtained, which, however, in my 
opinion, do not exceed the performance of a Wales ith 
properly amplified. With any of these lenses thus illumi- 
nated, the 29th and 380th bands of Nobert’s lines can be 
satisfactorily resolved; perhaps for this object the =,th of 
Messrs. Powell and Lealand does best, but they are all very 
nearly alike. Since reading the papers of Count Castracane 
and others (‘ Microscopical Journal,’ 1864, p. 249; 1867, 
p- 60, &c., &c.), I have carefully tried the violet ray obtained 
by a prism, but find that, although it possesses essentially the 
same qualities as that obtained in Von Baer’s method, the 
loss of light and the trouble of manipulation render it 
inferior for practical purposes. (Plate X.) 
Passing by the difficulty of manipulation—which might, 
perhaps, be overcome by proper mechanical contrivances—I 
limit myself here strictly to the question of loss of light. 
Besides the loss of light from reflection at the surface of 
the prism, there is a certain definite loss due to the disper- 
sion of the beam, a diminution which increases with the 
index of refraction of the prism, and also directly as the 
distance. Of course, therefore, the prism should be placed 
at as short a distance from the lower aperture of the achro- 
matic condenser as will permit sufficient dispersion to give a 
violet beam adequate to the homogeneous illumination of the 
instrument. I found, with a large flint-glass prism in my 
possession, that about eight inches’ distance was necessary 
for this purpose. ‘The results were very satisfactory to the 
eye, although with high powers I soon satisfied myself that 
the light was not so great as I had been in the habit of 
obtaining by transmitting the solar pencil through the am- 
monio-sulphate cell. To obtain a definite photographic 
comparison, I resorted to the following simple experiment. 
The solar rays were reflected by a plane mirror upon the 
prism, which was placed just outside the shutter of a 
darkened room. The arrangement was such as to throw the 
violet ray of the spectrum up a blackened tube into the dark 
room. At eight imches from the prism the violet light was 
intercepted by a concave amplifier, the mounting of which 
