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SUPPLEMENT TO THE 



and the angle of deviation from direct incidence in the 

 illumination (supposed to be the flame of a candle), found 

 requisite to shew their lines most perfectly measured : thus, 

 in figure 19, A, represents the flame of the candle behind 

 the stage, or locality of the object C, situated in the plane 

 D E 5 B the body of the engiscope ; G A its axis adjusted 

 for direct light ; the dotted line F K represents the axis of 

 the body; and H I the plane of the object adjusted for 

 the most perfect vision of the lines by oblique light. It is 

 evident that the angle A C K must be equal to the angle 

 F C G, which is the number of the degrees the axis of 

 the engiscope will be placed out of the axis of direct light 

 in order to exhibit the lines. 



My aplanatic engiscope, described in chapter 9 of this 

 work, may easily be made to measure angles of aperture. 

 All the extra apparatus required for this purpose will be 

 to cause the pin on which the stage is fixed to be truly 

 i. 19. 



