ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
511 
For a determined length of the rods, this formula gives the coefficient 
of expansion of the rods and thus the material of which they are to 
consist. On the other hand, when written in the form 
l= ±_-_y 
a — <p 
it gives for a determined material the length which must be given to the 
rods. 
The formula shows that the rods can be so much shorter, the more 
nearly equal are the coefficients of expansion of glass and setting, and the 
more widely different those of the rods and setting. As an example the 
author takes the case of a large telescope objective of 50 cm. diameter, con- 
sisting of a flint glass lens with coefficient of expansion y = 0*00000788, 
and crown glass lens with y = 0*00000954. The setting was of cast 
iron, with <f> = 0*00001061, while the rods were of zinc with a - 
0*00002918. 
For the flint, therefore, 
1061 - 788 _ 273 
1 ~ 2918 - 1061 “ 1857 
0*147. 
The radius of the flint was however 25 cm., so that the rods for the lens 
had to be 3*675 cm. long. 
For the crown, 
1061 - 954 _ 107 
1857 ”1857 
0*05762, 
and in this case the rods had to be 1 * 44 cm. long. The objective is seen 
in section and plan in the figure. 
The widest internal diameter of the setting at the part where the flint 
lens lay was 57 * 35 cm., while the diameter of the part where the crown 
glass lay was 52 * 88 cm. It is quite sufficient if three rods only, separated 
120° from each other, are placed between glass and setting. 
3), Illuminating- and other Apparatus. 
Hearson’s Biological Gas Incubator (fig. 44). — In this incubator 
the tank forming the water-jacket is made of stout copper, the outer case 
being of pine ; the space between it and the water-jacket is filled with a 
non-conductor of heat. The chamber is closed with an inner glass door 
and an outer wooden one. All the mountings are of bright lacquered 
brass, and the wood is stained and varnished. 
In the incubator, and immediately below the gas valve (fig. 45) 
which occupies the left-hand back top corner of the apparatus, is a small 
metallic hermetically sealed capsule, which contains a few drops of a 
liquid having a boiling point at or near the temperature at which the 
heated chamber is to be maintained. 
The capsule lies in a little holder suspended below the tube, through 
which the needle under the screw P passes. Soldered to the upper side 
