ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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1702. Really belongs to Part 1, and should be placed between Hart- 
soeker’s and Wilson’s screw-barrel Microscopes. Fig. 30, which is a back 
view, shows an oval wooden plate ; on the other side is a similar plate 
which holds the lens opposite the aperture A. Between the plates is a 
rotatory multiple object-holder (M N, fig. 31), the object being inserted 
in apertures in the circumference of the disc. Focussing is accomplished 
by means of the milled head B, which is attached to a screw regulating 
the distance between the two plates ; one of these carries the lens, and the 
Fig. 32, 
Fig. 38. 
other the rotary object-holder. The great point of interest is the rota- 
ting wheel of graduated diaphragms, ABODE (fig. 30), placed on the 
side of the object remote from the lens. This is the first instance of 
this useful and still surviving appliance. 
1710. We meet a crude estimate of aperture ; for Conradi says that 
the aperture of his object-glass was equal to a mustard seed. He also 
used a negative amplifier between the objective and eye-glass : this is 
the first notice of a Barlow lens. 
1715. (Fig. 32.) In Hertel’s Microscope the mount is of the 
“ telescope stand ” type, the inclination in arc being regulated by screw 
p 2 
