On the Use of Monochromatic Sunlight, &c. 
187 
copper are recorded. In one of these papers* he states that the 
ammonio-sulphate solution absorbs the red and yellow rays of the 
spectrum, and with them so much of the heat, that but “ twenty 
rays for every hundred that fell upon it ” were transmitted. 
In the 4 London, Edinburgh, and Dublin Philosophical Maga- 
zine’ for September, 1840,t the same gentleman published a paper 
“ On the Process of Daguerreotype and its Application to taking 
Portraits from the Life,” in which he describes his attempts to 
reconcile the chemical and visual foci of portrait objectives, to 
escape “ the effulgence ” of the solar rays thrown directly on the 
sitter, as practised at that time, “abstract from them their heat 
and take away from them their offensive brilliancy.” These are 
almost the very objects for which microscopists to-day resort to the 
copper solution. Professor Draper employed in his experiments 
“a large trough of plate glass, the interstice being an inch thick,” 
filled with a dilute solution of the ammonio-sulphate. Its size was 
about three feet square. This was so fixed in the course of the 
sun’s rays, reflected from a mirror upon the sitter, that his head 
and the adjacent parts were illuminated only by the light which had 
passed through the copper solution. By this device he reports he 
obtained excellent results. 
In the spring of 1869 I received a letter from one of the sons 
of Professor Draper (dated April 19 th), calling my attention to the 
above facts, and transmitting several daguerreotypes of microscopic 
objects, all bearing the marks of considerable age. These the 
writer (Prof. Henry Draper) states were made at various dates 
from 1851 to 1856. A Nachet microscope was used, and in every 
case the ammonio-sulphate of copper is said to have been employed. 
The results are not particularly good as compared with modern 
photo-micrographs, hut appear to me not much inferior to the best 
that could have been done by the daguerreotype method with the 
microscope used. The time was not yet ripe, and both microscopic 
objectives and photographic methods have greatly improved since 
those days. 
My present purpose does not permit me to give greater space 
to these reminiscences, the real object of this paper being to indi- 
cate the best practical method to be pursued in obtaining econo- 
mically the advantages of monochromatic sunlight for high-power 
definition. 
This object excludes a further consideration of the use of the 
prism. It does its work admirably, as I know by repeated trial, 
but the results are practically no better, even for photography, 
than those obtained by the use of the ammonio-sulphate cell; it 
requires greater skill to use, and the necessary apparatus is more 
expensive. For the same reason I shall say nothing in this article 
* Loc. cit., vol. sis., 1837, p. 473. t Vol. svii., p. 217. 
VOL. VIII. P 
