CAMERA LUCID^E 



279 



FIG. 218. 

 Beale's camera. 



greater normal diameter of the pupils of the eyes of some observers 

 in comparison with that of others'. 



Dr. Lionel Scale devised one of the simplest cameras, which has 

 the advantage of being thoroughly efficient. It consists of a piece 

 of tinted glass placed at an angle of 45 to the optic 

 axis, in the path of the emergent pencil. The idea 

 was first suggested by Amici, but he employed un- 

 colourecl glass ; Dr. Beale made it practical by the 

 employment of tinted glass. The first surface of the 

 glass reflects the magnified image upwards to the eye, 

 the paper and pencil being seen through the glass. 

 In its simplest form it is seen in fig. 218. The glass 

 is tinted to render the second reflection from the internal surface 

 of the glass inoperative. The reflection of the image is identical 

 with that of Soemmer ing's. 



Another camera lucida of soihje. merit is that devised by Amici, 

 and adapted to the horizontal microscope ^by Chevalier. The eye 

 looks through the microscope at the object (as in the ordinary view 

 of it), instead of looking at its projection upon the paper, the image 

 of the tracing point being projected upon the field an arrangement 

 which is in many respects more advantageous. This is effected by 

 combining a perforated silver-on-glass mirror with a reflecting 

 prism ; and its action will be understood by the accompanying 

 diagram (fig. 219). The ray a b proceeding from the object, after 

 emerging from the eye-piece of 

 the microscope, passes through 

 the central perforation in the 

 oblique mirror M, which is placed 

 in front of it, and so directly 

 onwards to the eye. On the other 

 hand, the ray a!, proceeding up- 

 wards from the tracing point, 

 enters the prism P, is reflected 

 from its inclined surface to the 

 inclined surface of the mirror M, 

 and is by it reflected to the eye 

 at b', in such parallelism to the 

 ray b proceeding from the object 

 that the two blend into one 

 image. 



A valuable and simple little 

 camera was devised by Mr. E. M. 

 Nelson in 1894. 1 It takes into F IG. 219. 



account the fact that while that 



form known as Beale's neutral tint (fig. 218) has been of great 

 value and persistence, it is yet a defective form ; the microscopic 

 image as received at the eye-piece is inverted and transposed. 

 Beale's camera corrects the inversion, while it leaves the transposi- 

 tion unaltered ; therefore all the objects drawn with this camera 

 are unlike the originals. In illustration place the letter p on 

 1 Journ. B, M. S, 1895, p, 21 et seq. 



