252 SUMMARY OF CURRENT RESEARCHES RELATING TO 
to science, and made pretensions to an acquaintance with magic. In 
1530 he wrote his treatise ‘On the Vanity of the Sciences,’ which was a 
caustic satire upon the inefficiency of the common modes of instruction. 
After an active, varied, and eventful life, he died at Grenoble in 1539. 
C3) Illuminating' and other Apparatus. 
New Measuring Apparatus for Microscopical Purposes.* — Dr. G. 
Lindau remarks that of all the pieces of apparatus which have been 
proposed for the measurement of small objects under the Microscope, the 
screw and glass micrometer in combination with objective or eye-piece has 
proved the best. Of these the eye-piece micrometer is by far the most 
convenient, and is to be preferred to all other micrometers, especially 
where a mean of several observations is taken. Cases however occur in 
which the eye-piece micrometer fails to be of service, as in the measure- 
ment of thin membranes or threads and in physical investigations on wave- 
lengths of light, &c. A micrometer constructed by Dr. V. Wellmann 
may replace it with advantage in these cases. It was originally intended 
for astronomical purposes, but forms a very useful micrometer for the 
Microscope. It is especially serviceable for measuring very small objects 
not exceeding a few fx in size. It differs in principle from all other 
micrometers in depending on the double refraction of light in certain 
crystals. It is well known that on looking at a point through a prism 
of rock-crystal two images, the ordinary and the extraordinary, are seen. 
As the prism is rotated about the optic axis, the extraordinary image 
rotates about the ordinary. Consequently, if such a prism is fitted over 
a microscopic eye-piece in whose focus a thread is stretched, two images 
of the thread are seen. On rotating the prism the apparent distance of 
these two images for a certain position becomes zero (the images coincide), 
and on rotating through 90° from this position it reaches a maximum. 
On continuing the rotation up to 180° the images agaiu coincide. In the 
rotation from 180° to 360° the images behave in a similar way, except 
that the movable one changes over to the other side. 
In the new micrometer these two images are used in precisely the 
same way as the threads of a screw micrometer, for by suitable rotation 
of the prism their distance is made equal to the image of the object to 
be measured. The distance of the two images is given by 
A — m sin <£, 
where is the angle through which the prism is rotated, and m is the 
apparent maximum distance of the images for a given magnification v. 
This constant m is easily determined by measuring objects of known 
size. 
Now the apparent magnitude of an object, of which the actual size d 
is to be determined, is given by 
A ' = d . v. 
Consequently, when by rotating the prism 
A' is made = A = m sin cj > , 
* Naturwissensch. Wochenschr., iv. (1889) pp. 185-6 (1 fig.) 
