On a Series of Lantern Slides. By Dr. A. Clifford Mercer. 313 
of wood with a screw-eye at its free end. A lever-rod fastened by a 
ring and milled-headed screw to the coarse-adjustment wheel drops 
into the screw-eye. By this simple contrivance coarse focusing can 
be done on the ground glass at any distance from the Microscope — 
by moving the long brass rod in its support, and so moving the block 
and lever. From about the fine-adjustment wheel (fig. 35), an endless 
cord passes down and about a pulley-wheel, actuated by a brass rod, 
supported all along under the camera bed, so that fine focusing can be 
done at any distance from the Microscope. Coarse focusing is done on 
the ground-glass, fine focusing by a focusing lens held against a piece 
of polished plate-glass substituted for the ground-glass. 
Projecting from the sliding block is a second bar ending with a 
screw-eye which receives a lever from the rack and pinion moving 
the draw-tube. 
Either lever is quickly fastened or loosened by a thumb-screw 
closing or opening the ring about the milled wheel. First one 
lever alone, or the other, is used ; never the two together. The 
draw-tube lever is used when by the Woodward method * the back 
focus of an objective is extended by an achromatic meniscus screwed 
into the objective end of the draw-tube. It seems to be almost 
forgotten that Dr. Woodward produced his famous photomicrographs 
by this method. The method, briefly, is first to arrange the object 
and illumination, to adjust the objective-collar most carefully for 
the 10-in. distance, then to remove the eye-piece, to screw the 
meniscus into the objective end of the draw-tube, and, finally, to 
adjust the draw-tube until the meniscus is at the right distance 
from the back of the objective to extend the focus to the ground 
glass, the objective meantime remaining undisturbed in its best 
condition. No other method gave as good results until the apo- 
chromatic method was introduced. In the apochromatic method, 
however, the objective is not left undisturbed, and the projecting 
eye-piece has two lenses with four reflecting surfaces. If it were 
practicable to make a compensating concave with only two reflecting 
surfaces, analogous to the Woodward meniscus, to be used instead 
of the projecting eye-piece, a so modified apochromatic method would 
be more rapid and might perhaps give even more brilliant results. 
The lamp, edge of the flame on, and bull’s-eye are carefully 
centered on sliding boards, so as to be able conveniently to use 
Mr. Nelson’s method of illumination.! The light (fig. 35) passes 
through an alum or water cell, and an ammonio-sulphate of copper cell 
— seen between thebull’s-eye and substage condenser — to eliminate most 
of the heat-rays, to give visually monochromatic light, and to make the 
visual image approximately the same as the actinic image. I believe 
by using the blue cell I have been able to approximately focus the 
actinic rays ; for I have used very various objectives with the blue 
This Journal, 1879, p. 664. 
f Op. cit., 1885, p. 713. 
