386 A. S. Johnson on an Indicator Stage for Microscopes. 
Arr. XLII.—An account of an Indicator Stage for Microscopes ; 
; by A. 8. Jounson, Albany, N. Y. 
In October last Messrs. Grunow of New Haven completed for 
me and adapted to my microscope a moveable stage, upon the 
principles stated by Prof. Bailey in his article upon the Universal 
Indicator. Carrying out his suggestions, they have succeeded in 
overcoming the difficulties which were pointed out as likely to 
attend attempts to apply the indicator to moveable stages. The 
indicator stage combines all the convenience of the best move- 
able stages, with all the advantages to its possessor of the univer- 
sal indicator. By means of it all parts of a slide can be rapidly 
brought by equal sweeps under examination and the noteworthy 
objects registered. The registration is identical with that ob- 
from side to side; the top plate moves forward and back 1th 
50ths of an inch, which extend from the middle of the side 
edges towards the centre of the stage. That on the left is 
graduated from 0 to 50; that on the right from 60 to 110. In 
the middle plate are two similar scales, extending from the middle 
of the front and rear edges of the plate towards its centre. ‘The 
front scale is graduated from 20 to 50, the other from 50 to 80. 
_ Over each scale projects a steel index attached to the plate above, 
to facilitate accurate reading of the scales. Upon the top plate 
is engraved a horizontal line passing across the stage, intercepted 
of course by the hole in the centre, but so drawn that continued 
across the hole it bisects horizontally the field of view. At right 
angles to this line two others are engraved each crossing it on 
opposite sides of the centre at the distance of one inch from the 
centre of the field of view, which is also the centre of the stage. 
The position of the several scales is adjusted. with reference to 
these lines as follows. When the horizontal line, continu 
across the hole in the stage, bisects the field of view, the indexes 
attached to the front and rear edges of the top plate stand at 50 
on the two scales of the middle plate. As upon the universal 
indicator the horizontal line at 50 also passes through the centre 
of the field of view, it is obvious that the positions of that line 
and of the horizontal line on the stage are identical. 
The verticals being each an inch distant from the centre of 
the field of view at their intersection with the horizontal line, 
and one inch being also the distance on the universal indicator 
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