114 Modern Microscopy 



which the object is viewed in the ordinary way. The 

 prisms are so set that the image of the paper on which the 

 drawing is to be made, and which is reflected by a mirror 

 to the prisms, is by them conveyed to the eye. Thus the 

 pencils of light reach the eye coincidently from both the 

 microscope and the paper, and when drawing the object 

 the pencil-point appears in the field of view very distinctly, 

 and the minutest details can be exactly traced. Low- 

 power eyepieces should be used with this camera lucida. 

 There is another very commonly-used form of camera 

 lucida, invented by Wollaston. In employing this it is 

 necessary to put the microscope in a horizontal position,' 

 but it is somewhat difficult to see the pencil-point with it 

 when working. To do this it is necessary that one part of 

 the pupil of the eye shall be over the prism and receiving 

 the image, while the other part is looking down on the 

 paper below. A little practice soon enables one to do this. 

 Some workers prefer this to Beale's neutral-tint reflector, 

 but in our hands no superiority has been apparent in it. 

 There are many other forms peculiar to individual makers, 

 possessing more or less merit, some of which may be used 

 with the tube in any position, the Swift-Ives pattern, made 

 by Swift and Son, being particularly efficient ; it can be 

 used with the microscope set vertically or inclined. 



The Measurement of Objects. 



There are three ways in which this may be effected : 



1. By having the stage divided — applicable to mechani- 



cal stages only. 



2. By means of a camera lucida and a stage micro- 



meter. 



3. By means of eyepiece and stage micrometers. 



1. If the movements of a mechanical stage are divided 

 and read by verniers to very small parts of an inch or 

 millimetre, the measurement of an object can be effected by 

 having in the eyepiece a disc of glass with a diamond-cut 



