NOTES AND MEMORANDA. 
205 
position as to concentrate tlie light, at the angle above indicated, upon 
the frustule under examination. After this nothing remains hut to 
regulate the cover correction and the fine adjustment. The precise 
angle which should be given to the illuminating 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 especially suitable 
for photographing the Amphipleura, but the simpler method above 
described answers every purpose if the object is to compare objectives. 
An Optical Instrument of Mr. Tolles’s Device.— The ‘ Lens ’ 
states that Mr. Tolies has just made to order a novel instrument. It 
consists of a vertical tube, seven-tenths of an inch in diameter inside, 
twelve inches high, supported on a base about two inches square. On 
one of the vertical faces of the base is a compound achromatic telescope 
lens three-quarters of an inch in diameter. Between the lenses is a 
total reflecting prism ; at the upper end of the tube a solid ocular. 
Looking down the tube, the observer can obtain a well-defined image, 
magnified thirteen diameters, of an object distant three feet horizon- 
tally from the lens. “ As the term micro-telescope has been applied to 
Tolles’s instruments where one tube and ocular serves for both 
microscope and telescope, this instrument may be called a tele- 
microscope.” 
The Paraboloid an Immersion Instrument— Notwithstanding 
the introduction into use of special contrivances as immersion para- 
boloids, it may not have occurred to all who use the microscope that 
the ordinary form of parabolic illuminator is capable of being used 
wet with excellent results. Placing the microscope in a vertical 
position, says T. D. B. in the 1 American Naturalist ’ for August, and 
greasing the rod in the centre of the paraboloid to keep the water 
from running out by the side of it, the cup of the paraboloid is filled 
with water heaped up as far as can be without running over, and then 
brought up until the water comes in contact with the under surface of 
the slide. The direction of the rays leaving the paraboloid is not 
altered by this arrangement, but dispersion at two surfaces is avoided 
and the rays enter the object-slide without the usual refraction, and at 
such an angle as to suffer total internal reflexion before reaching the 
objective. With the highest objectives generally used with black- 
ground illumination, as a ^th of 75° to 110°, the object seems no 
brighter than usual, but the field is free from the foggy diffuse light, 
otherwise present, and the object appears, beautifully distinct, upon a 
jet-black ground. Even a itli or ith of 130° gives the same effect of 
a deep black background, and shows the object with good stereoscopic 
effect in Wenham’s binocular. With objectives of 170°, the main 
effect is that of a dark background, though not so perfect as with the 
lower angles. 
