NOTES AND CORRESPONDENCE. 
Monochromatic Illumination—Last year, in the October 
publication of your most excellent ‘ Journal of Microscopical 
Science,’ which is forwarded to me by M. Bailliére, of Paris, 
was published a letter from the Signor ab Count Francesco 
Castracane to the R. P. Secchi, about a certain mode of illu- 
mination, named a monochromatic one, and intended for the 
use of the microscope. One month after, a French review 
(the ‘ Cosmos’) gave an account of a communication which 
had been made to the Institute about their mode of observing 
the microscopic phenomena. At last, in the first number of 
your journal for this present year, Mr. Barkas, of Newcastle- 
on-Tyne, has spoken also of this, Count Castracane’s new 
mode of illumination. On that matter I wrote to these gen- 
tlemen, and received from both of them most kind letters. 
We have since interchanged some specimens of diatoms, and 
to-day I am happy to number these two learned micrographers 
amongst my correspondents. 
But as to the homogeneous, monochromatic light, its use 
for the microscopic student is far from being new. Indeed, 
I have employed that light since it has been made known to 
me for this use by Amici; that is to say, eleven years since. 
However, when I perused the communication of Mr. Barkas 
and Count Castracane, my first thought was that a new mode 
of illumination was at stake, especially for the most hardly 
resolvable tests, and particularly the Nobert lines. I there- 
fore begged some explanation from those two gentlemen. 
It is certain that, using Amici’s refracting prism, by means 
of which light is sufficiently dispersed, one may detect, with 
a very feeble objective, what the strongest could not allow to 
suspect with the direct white light. With the aid of such a 
prism, I have been able to view the specimens, for which I 
am indebted to the kindness of Mr. Barkas, together with 
some others, and to determine perfectly the strie of the 
