been secured. It often happens that the precise tint required 

 can not be secured at all. in any case by this method there is 

 always being inserted between the eye and the object, various 

 pieces of apparatus which can not be regarded as other than 

 necessary evils. It is needless to go into the particulars of 

 instruments made by different makers. What has been said has 

 a general application. A second defect presented by many of 

 the camera lucidas, is the double reflection due to a silvered mir- 

 ror. The thickness of this mirror has been carefully adjusted in 

 order, as far as possible, to superimpose the various. images one 

 on the other, but it is impossible to get rid of the double reflec- 

 tion and while this can be tolerated for a short time, if the in- 

 strument is in use for a considerable length of time, say several 

 hours, this double reflection of the pencil point becomes very 

 tiresome to the eye. 



Any form of camera lucida is an instrument well-calculated 

 for the destruction of eye-sight. The writer has during 

 many years of experience been endeavoring to reduce the 

 injury to eye-sight in connection with the use of the camera 

 lucida, and the following suggestions, embodied in the out- 

 fit here described, are the result of his experiences. In the 

 first place, he has substituted for the ordinary mirror a 45° 

 prism. Fig. i, 24. The advantages obtained by this substitu- 

 tion are as follows: i. The prism may be of any desired size 

 so that it may be mounted at a considerable distance from 

 the eye-piece of the microscope. This secures an increased 

 magnification of the drawing, and the advisability of this in- 

 creased magnification will be dwelt upon on a subsequent page. 

 2. A second advantage in the use of the prism as a reflector is 

 the disappearance of the double reflection, and the securing of a 

 total reflection. The Hght passes from the drawing-point 

 through the lower face of the prism in a nearly perpendicular 

 direction and with very little loss. 'It is then totally reflected 

 from the oblique face and passes outward nearly at right angles 

 to the vertical iace, again with very slight diminution. The 

 loss of light is therefore considerably less than in the case of 

 the usual mirror, in addition to the securing of a totaf reflection 

 destitute of doubles. 3. A third advantage and one of consider- 

 able importance is the stabiHty of the apparatus here described ; 

 it rarely gets out of register. 



The second modification is that wliich lias been referred to on 

 a previous page, as the blind worked by foot-power. Fig. i, 32. 

 The object of this blind is to illuminate the drawing with any de- 

 gree of light at an instant's notice and to do' this without in any 

 way disturbing the adjustment of any part of the microscope or 



