188 
On the Use of Monochromatic Sunlight, 
cn 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 Amphi- 
pleura pellucida as a Test-object for High Powers.” * 
“ Erect a perpendicular wooden screen about two feet square on 
one edge of a small table. Cut in this a circular hole an inch and 
a half in diameter at about the height of the under surface of the 
stage of the microscope. On the outside of this hole mount a 
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 through 
which the direct rays of the sun can fall upon the mirror, and 
adjust this so as to throw the solar pencil nearly horizontally 
through the ammonio-sulphate cell. The mirror and achromatic 
condenser, if the microscope has one, are removed and the micro- 
scope turned so that the solar pencil shall fall with the desired 
degree of obliquity on the under surface of the object. It will 
generally be best to condense the light upon the object by a small 
ordinary lens, or still better by a low-power objective mounted like 
a bull’s-eye lens on a separate stand. 
For prolonged observation, however, the motion of the sun will 
render it necessary to readjust the mirror from time to time, and 
the use of a heliostat becomes desirable. This gives the most 
satisfactory results no doubt, but the cost of the heliostat will, of 
course, prevent it from coming into general use, except among 
those who desire to photograph what they observe, and the simpler 
methods above detailed will answer very satisfactorily for every 
other purpose. 
The ammonio-sulphate cell used in either method should be 
* ‘American Naturalist,’ April, 1872, p. 193. 
