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MONOCHROMATIC SUNLIGHT. 457 
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 dmmonio-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 
on the use of artificial lights, further than that both the prism and 
the ammonio-sulphate cell may be satisfactorily used with either 
the Calcium, the Magnesium or the Electric lights by those who 
are unable conveniently to secure the advantages of sunlight. The 
light of ordinary coal oil or gas lamps, however, is not suitable 
for the purpose. 
Two very simple methods of securing the advantages of the 
ammonio-sulphate solution will now be briefly described. 
(a.) I suppose the observer to be possessed of a good micro- 
Scope stand, with achromatic condenser and suitable objectives. 
Then it is only necessary to prepare a proper ammonio-sulphate 
cell and fix it between the plane mirror of the instrument and the. 
achromatic condenser. The microscope should be set near a 
window so that the direct rays of the sun fall on the plane mirror, 
While the head of the observer is protected by a convenient screen 
and all becomes easy. 
(b.) A still better method for the resolution of lined test- 
objects with the highest powers, and one which is almost as simple 
as the foregoing is that described in my paper “On the use of 
Amphipleura pellucida as a test-object for high*powers.” (This 
Journal, April, 1872, p. 193.) 
“ Erect a perpendicular wooden screen about two feet square on 
= Me edge of a small table. Cut in this a circular hole an inch and 
_ half in diameter at about the height of the under surface of the 
Stage of the microscope. On the outside of this hole mount 
à small plane mirror which can be adjusted by passing the hand 
to the outside of the screen. On the inside cover the hole with 
the ammonio-sulphate cell.” Now move the table to a window 
ugh which the direct rays of the sun can fall upon the mirror, 
and adjust this so as to throw the solar pencil nearly horizontally 
ugh the ammonio-sulphate cell. The mirror, and achromatic 
condenser, if the microscope has one, are removed and the micro- 
“ope turned so that the solar pencil shall fall with the desired 
degree of obliquity on the under surface of the object. It will 
Senerally be best to condense the light upon the object by a small 
