70 



THE MICROSCOPE 



Vertical 



camera 



lucida. 



Abbe 



camera 

 lucida. 



i 



I 



<^- 



(Ar 

 ( 



Upwards into his eye by means of a reflection in the prism from 

 a half- silvered surface. The eye also sees the paper and pencil 

 through the half- silvered surface, and can draw the object 

 seen through the microscope accurately and rapidly, because it 

 appears to be superimposed on the paper. 



If the eyepiece of the microscope is closer to the paper than 

 about 10 inches (the near point of vision), the pencil will not 

 appear sharp ; and to obviate the necessity of raising the micro- 

 scope, a lens is supplied below the prism which enables the pencil 

 and paper to be clearly seen at a distance of about 6 inches, which 

 is the usual height of a microscope body. The lens is also a 

 great assistance even when the paper is 10 inches away. It 

 fits into a recess in the mount and is held in by a turn-button. 



The Beck horizontal camera lucida is superior to the old 

 WoUaston form, as the eye does not require to be held in an 

 exact position during the process, and there is no training required 

 for its use. The^only^care that is required is to see that neither 



the illumination of 

 the object nor the 

 paper is so brilliant 

 as to obscure the 

 one or the other. 

 The relative illumi- 

 nation can be easily 

 regulated by a neu- 

 tral tint glass placed 

 either between the 

 prism and the paper 

 to reduce the apparent brightness of the paper, or between the 

 microscope eyepiece and the prism to reduce the apparent bright- 

 ness of the microscope image, or the illumination of the micro- 

 scope may be varied by any of the means previously referred 

 to. A slot is provided in the two positions to receive the neutral 

 glass. 



The Beck vertical camera lucida (Fig. 78) is a prism which 

 acts in a similar manner except that the microscope must be 

 placed in a vertical or an inclined position. When the microscope 

 is in a vertical position, the drawing paper must be placed on a 

 slanting board at an angle of 30° in front of the microscope. 

 In other respects the manipulation is the same. When the 

 instrument is used in an inclined position, the tube of the micro- 

 scope must be set at an angle of 60° and the paper may be placed 

 upon the table. The same arrangements are made for the 

 reception of the lens and neutral glass. 



The Abbe camera lucida (Fig. 79) consists of a prism over the 

 eyepiece and a large mirror placed a few inches to one side 

 in a horizontal direction. The prism has a completely silvered 

 surface, with a small aperture in the centre, and is not so easily 



Fig.' 78.- 



-No. 3369, Beck Vertical Camera 

 Lucida. 



