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 pacer 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 yoniometers *" 

 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 



Fig . 30. Fig. si. is shown . in % 31 > so ^ 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 F, 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 yovv (yonu), knee. 

 58 



