512 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the Washington Microscopical Society. This form is adapted from the 
more complicated and expensive forms used by microscopists, and is 
claimed to be especially useful from the fact that it can be made at a 
trifling cost by any one possessing a little mechanical skill. In fig. 56, A 
represents the wooden block or stage which is fastened upon the brass 
stage of the Microscope. A space is cut from the upper surface of 
this block, as shown by C, into which is fitted a piece of copper 
plate (B, B', B"). A round hole is also cut at F, the opening of 
the brass stage, to allow of the illumination of the object to be 
examined. The slide is placed on the copper bed with its ends 
resting at B' and B", as indicated by the dotted lines. The heat is 
applied by a spirit-lamp at the end L of the copper plate B which 
Fig. 56. 
gradually transmits the heat by conduction to the slide. The tempera- 
ture is registered by the thermometer E, which is screwed fast to the 
copper plate. 
Application of Apertometer to the Microscope.* — Herr Kayser 
remarks that the narrowest perceptible distance of a wave-length stated 
by Fraunhofer and Nobert, is not the extreme limit which the newer 
Microscopes with oil-immersion systems have reached in the resolution 
of the structure of diatoms. The smallest recognizable distance e 
approximates to the expression established theoretically by Helmholtz 
A 
e = , 
2 sm a 
where A denotes the wave-length and a the angle of divergence under 
which the extreme rays from the axis of the object fall upon the objective 
system. Since this angle can with an immersion lens be nearly a right 
angle, the numerical expression for the limit, taking A = 0 • 00055 mm. in 
the most general case, will amount to the half wave-length 0 • 000275 mm. 
According to the practical investigations of Abbe and Dippel, the re- 
solving power of an objective system stands in very close relation to the 
magnitude of the angle of divergence. On this account makers are 
* Schrift. der Naturforsch. Gesells. Danzig, vii. (1890) pp. xiii.-xvi. 
