HN SEBO OR Ee DUES Be ae BAL PEM ee mk St Reece Teel my Ae 
Aus re Nees RTEA Misti er 
MONOCHROMATIC SUNLIGHT. 455 
“On Monochromatic Illumination.” These remarks were sug- 
gested by the perusal of a letter from Count Francisco Castracane 
published in the same journal some time before. (Ibid. vol. v, 
1865, p. 249.) 
4 Count Castracane’s method consisted essentially in the use of a 
= _ prism by which the sunlight was decomposed, and any selected 
q color could be employed, blue or green seeming to him most 
advantageous. Mine consisted in passing the sunlight through a 
` cell containing a saturated solution of the sulphate of copper in 
ammonia, which transmits a bluish violet light, admirably suited 
to high power definition and less fatiguing to the eye than any 
other color. 
At the time I supposed Count Castracane’s method to be new ; 
the one I employed I ascribed to Von Baer (“Einleitung in die 
Höhere Optik” p. 48). I have since learned that I was in error in 
both particulars. The proposition to escape chromatic aberration 
by employing monochromatic illumination goes back in fact to a 
very remote period in the history of achromatic microscopes, and 
monochromatic lamps, as well as the use of the prism and of 
glasses and colored fluids as absorptive media, were early suggested. 
It would carry me away from my present purpose to go into a de- 
tailed history of the various attempts made from time to time in 
these directions. As the construction of achromatic objectives 
continued to improve, these devices fell into obscurity and it is 
only of late that attention has been directed to, them anew. As 
for Count Castracane’s method, without going further back, a 
full account of all the principles involved in the tse of the prism 
for attaining monochromatic light to illuminate the microscope 
will be found in Chapter vii of the article on the microscope in the 
eighth edition of the “ Encyclopedia Britannica” (American edi- 
tion 1857, Boston, Vol. xiv, p. 798). 
The use of the solution of the ammonio-sulphate of copper to 
exclude certain portions of the solar rays especially for photo- 
graphic purposes, would appear to have been first suggested by. 
one of our own countrymen more than thirty years ago. 
: Professor J. W. Draper published in the “Journal of the Frank- 
oi Institute” of Philadelphia, during the year 1837, a series of 
Experiments on Solar Light” in the course of which several ob- 
Servations on the properties of the ammonio-sulphate of copper 
are recorded. In one of these papers (Loc. cit. Vol. xix, 1837, p- 
