AMPHIPLEURA PELLUCIDA. 193 
results, however, are attained by the light of the electric lamp or 
of the sun rendered monochromatic by passing through a satu- 
rated solution of the sulphate of copper in strong aqua ammonize 
and of about the eighth of an inch in thickness. 
Of these methods, that by sunlight involves least trouble and 
expense, and may be best managed as follows :— Erect a perpen- 
dicular 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. (A piece of dark blue glass will answer the pur- 
pose though not so well.) Now move the table to a window 
through which the direct rays of the sun can fall upon the mirror, 
adjust this so as to throw a nearly horizontal pencil of parallel 
rays through the hole, and place the microscope in the shade of 
the screen in such a position that the parallel blue rays will fall 
on the under surface of the amphipleura slide at an angle of from 
fifty to seventy-five degrees with the plane of the slide (I sup- 
pose the frustule to be examined has first been found by ordinary 
day-light or lamp-light). Next place a small bull’s eye or any 
other condenser of from one to three inches focal length (mounted 
©n à separate stand or on a radial arm) in the parallel pencil in 
Such a position as to concentrate the light, at the angle above 
. 
dicated, upon the frustule under examination. After this noth- 
ing remains but to regulate the cover correction and the fine 
adjustment. The precise angle which should be given to the 
uminating pencil will vary with the angle of aperture of the 
objective used. As a rule it should be less than half the angle of 
aperture of the objective, and 70° to 75° is the maximum angle 
Which should be given even for objectives of 170° angle, a greater 
angle, distorting the image without improving the definition. 
The same results can be obtained by using a heliostat to fix the 
direction of the solar rays, and obtaining obliquity by an achro- 
‘Matic condenser of from 130° to 150° suitably decentred. On 
Account of the stability of the illumination this method is espec- 
tally suitable for photographing the Amphipleura, but the simpler 
oR above described answers every purpose if the object is to 
Compare objectives, 
