ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
877 
1/5 mm., and every millimetre is marked by a figure. For readings 
below 1/5 mm. each, instrument is provided with a micrometer-Micro- 
scope which is so arranged that two complete turns of the screw on the 
eye-piece correspond to one division of the scale, so that one division 
of the screw-head, which is divided into 100 equal parts, represents 1 /x . 
In the two instruments with contact adjustment the contact is effected 
by means of an agate pointer with spherical polished termination. 
The contact micrometer represented in fig. 100 is intended to measure 
thicknesses up to 50 mm. by contact adjustment. The arm A, screwed 
to the base-plate of the apparatus, serves to support the Microscope, and 
also the arrangement for raising and lowering the scale. The platinum 
scale M, about 60 mm. long, is suspended at its upper end right and 
left between two points S. The lower end rests at the back against a 
projecting pin which is adjustable, so that the plane of the scale can 
be brought exactly into the line of displacement. This is the case 
when all the divisions of the scale during a complete displacement 
appear equally distinct, as seen in the Microscope. On the stand is a 
scale, divided in centimetres, on which the position of the zero point of 
the movable scale can be read with the naked eye. A smooth movement, 
as free as possible from friction, is obtained by means of two steel 
cylinders F x and F 2 , which pass through two three-sided openings. 
The mechanism for raising and lowering the scale is easily under- 
stood from the figure. A cord fastened to the upper end of the cylinder 
F x passes over the pulley K to the grooved wheel J, which is turned by 
the handles H 1 and H 2 . The weight of the cylinders and scale is partly 
counterpoised by the small weight G. 
The object of which the thickness is to be determined is placed on a 
glass plate, 7 cm. in diameter and 1 cm. thick, let into the base-plate of 
the apparatus. 
The second instrument, the Comparator, represented in fig. 101, serves 
for the measurement of divisions, gratings, spectra, star photographs, &c., 
up to 100 mm. It differs from the last in that the length to be measured 
is adjusted optically by a Microscope instead of by contact. 
On a short strong tripod (fig. 101) is screwed a base-plate 16 cm. long, 
and about a hand broad. This carries the supports for the two Micro- 
scopes I. and II., of which I. is adjusted upon the scale, and II. upon 
the object. Scale and object are attached to a slide dovetailed into the 
base-plate, and displaceable from right to left. 
Large displacements are made by hand by means of the knob on the 
right. The finer movement is effected by the screw S r The scale M 
is brought directly upon the slide A A. The object on the other hand 
lies on a second special slide B, which is connected with the main slide 
in such a way that it partakes of all the displacements of the latter, but 
can also be moved separately by means of the screw S 2 on the left. 
For the observation of objects with transmitted light, half of the 
main slide near the object slide is cut away along its whole length. 
The illumination in such cases is by means of a mirror beneath the base- 
plate. Further, in order to render possible the examination of square 
and circular plates, the support of Microscope II. is curved out so far 
that the foot is more than 50 mm. from the middle of the slide. 
For many purposes it is useful to fit upon the slide B a rotating plate, 
