THE MICROSCOPE. 
piece, object-piece, and stage, a corresponding change would be 
made in the magnifying power of the instrument, and a conse¬ 
quent change in the dimensions of the picture of any object 
projected by the camera on the paper, though no change be made 
in the distance of the paper from the camera. 
In fine, the method of measuring the actual dimensions of a 
microscopic object by means of a scale drawn with the aid of the 
camera, requires that the instrument and the paper shall be in 
precisely the same state when the image of the object is projected 
on the paper as they were when the scale was drawn upon the 
paper. 
If this condition be observed, measurements can be made by 
the camera with all the necessary facility and precision. 
62. In microscopic researches it is frequently necessary to 
measure the angles at which the lines which form the contour of 
objects are inclined to each other. Various forms of goniometers * 
have been contrived for this purpose. One of the most simple 
and convenient of these consists of a circular plate of brass c c, 
fig. 30, having a central opening in which a disc of glass is set, 
on which a diameter d b is engraved with a diamond point. 
Upon this, and concentric with it, another similar plate, toothed 
at the edge, is placed, having also a disc of glass of the same 
magnitude set in it, with a diameter a c in like manner engraved 
upon it. Upon the plate c c an ear is cast, in which a pinion is 
inserted, which, working in the teeth of the second disc, gives it 
a motion round its centre, by which the diameter a c is made 
successively to assume all possible angles with the diameter d b. 
This piece is inserted in the eye-piece a b, a side view of which 
is shown in fig. 31, so as to be 
concentric with the lenses, and 
to coincide with the focus of 
the eye-lens. The lines a c 
and b d will then be seen pro¬ 
jected on the image of the 
object, and if the vertex of the 
angle it is desired to measure 
be brought, by means of the stage adjustments, to coincide 
with the centre o of the disc abed , where the two engraved 
diameters intersect, and so that one side of the angle to be 
measured shall coincide with the fixed line d b , the line a c can 
be turned by the pinion e, until it shall coincide with the other 
side. A graduated circle which surrounds the disc will then 
show the magnitude of the angle at which b d and a c are inclined. 
* From the Greek word y 6vv ( gonu ), knee. 
5.8 
