ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
G21 
single by means of a small protractor, and screwed on. The woodcuts 
are drawn to exactly half size so that wood patterns may be easily made 
from them. In making the patterns for the brass castings, the female 
dovetails as in figs, c and d should be left solid. The slot shown in 
fig. 60 should be cut with an ordinary circular saw and screws inserted 
«o that the dovetails can be pulled down to fit the male slides. In 
fig. 61 a is shown the optical body and draw-tube. 
A light ring about 3/8 in. from the top is soldered upon the draw- 
tube, and two stops to prevent internal reflections are fitted inside. The 
upper stop is fitted about 1/8 in. below the total length of the body 
Fig. 60. Fig. 61. 
of the eye-piece when this is inserted in the tube ; the second stop, with 
aperture of about 6/10 in. in diameter, is nearly at the lower end. At 
the end of the body-tube a brass casting is soldered, which must be 
screwed to the standard of the Royal Microscopical Society. The total 
length of the optical tube should be 160 mm. To the body is soldered 
a gun-metal dovetail (fig. 61 o) which slides into the upper part of 
fig. 60 a. A similar slide is soldered on to a tube as in figs. 62 a and b, to 
form the main portion of the fine-adjustment. A section of the fine- 
adjustment is seen in fig. 63. The screw should be about 50 threads 
to the inch, and is made out of No. 20 pianoforte wire ; its point bears 
upon a hardened steel stud. 
The rack-and-pinion coarse-adjustment is of the spiral form originally 
patented by Messrs. Swift and Sons. A piece of steel pinion wire of 
12 leaves and 2J tenths in diameter is taken, and the ends hammered 
flat so that one can be held in a bench vice and the other clamped by a 
hand vice ; the pinion wire is then twisted through a right angle, when 
1894 2 U 
