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YI. — On the Use of Monochromatic Sunlight, as an Aid to 
High-power Definition. 
By Dr. J. J. Woodward, U. S. Army. 
A few years ago I published some brief remarks “ On Monochro- 
matic illumination.” * These remarks were suggested by the 
perusal of a letter from Count Francisco Castracane published in 
the same Journal some time before.f 
Count Castracane’s method consisted essentially in the use of a 
prism by which the sunlight was decomposed, and any selected 
colour 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 hi 
ammonia, which transmits a bluish violet light, admirably suited 
to high-power definition, and less fatiguing to the eye than any 
other colour. 
At the time I supposed Count Castracane’s method to be new ; 
the one I employed I ascribed to Yon Baer 4 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 achro- 
matic microscopes ; and monochromatic lamps, as well as the use of 
the prism and of glasses and coloured fluids as absorptive media, 
were early suggested. It would carry me away from my present 
purpose to go into a detailed 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 
farther back, a full account of all the principles involved in the 
use 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 ‘ Encyclopaedia Britannica.’ § 
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 W. J. Draper published in the c Journal of the 
Franklin Institute ’ of Philadelphia, during the year 1837, a 
series of “ Experiments on Solar Light,” in the course of which 
several observations on the properties of the ammonio-sulphate of 
* The then Journal of the Royal Microscopical Society, vol. vii., 1867, p. 253. 
f Ibid., vol. v., 1865, p. 249. J ‘ Eiuleitung in die Hohere Optik,’ p. 48. 
§ American edition, 1857, Boston, vol. xiv., p. 798. 
