l62 



DRAWING WITH A CAMERA LUCIDA 



CCh. VI 



reflected twice, and the image appears as when looking directly into 

 the microscope. In others the rays are reflected but once, and the 

 image has the inversion produced by a plane mirror. For drawing 

 purposes this inversion is a great objection, as dt is necessary to 



similarly invert aU the details 

 added free-hand. 



(2) By a camera lucida re- 

 flecting the rays of Ught from 

 the drawing paper, etc. so 

 that their direction when they 

 reach the eye coincides with 

 the direction of the rays from 

 the microscope (fig. 100). In 

 all of the camera lucidas of 

 this group, the rays from the 

 paper are twice reflected and 

 no inversion appears. 



The better forms of camera 

 lucidas (WoUaston's, Gru- 

 now's, Abbe's, etc.) may be 

 used for drawing both with 

 low and with high powers. 

 Some require the microscope 

 to be incUned (fig. 99) whUe 

 others are designed to be used 

 on the microscope in a vertical 

 position. As in biological 

 work, it is often necessary to 

 have the microscope vertical, 



Fig. 99. Wollaston's Camera LucrDA. 



Axis The optic axis of the microscope. 



Ocular The upper end of the ocular. 



A, B Two rays outside the axis to show 

 that they cross twice and hence have the 

 same relative position as when they emerge 

 from the ocular. 



Camera lucida The quadrangular piece of 

 . glass giving the double internal reflection to 

 change the direction of the axial ray 90°. 



CD, AB The virtual image, drawing paper 

 and pencil partly overlapping. Where they 

 overlap the appearance is that of one field. 



the form for a vertical microscope is to be preferred (see fig. 100). 



§269. Avoidance of distortion. — In order that the picture 

 drawn by the aid of a camera lucida may not be distorted, it is neces- 

 sary that the axial ray from the image on the drawing surface shall 

 be at right angles to the drawing surface (fig. 99, loi). 



§ 270. Wollaston's camera lucida. — This is a quadrangular 

 prism of glass put in the path of the rays from the microscope, and 



