Some Recent Forms of Camera Lucicla. By Frank Crisp. 23 



the extreme rays make an angle x the complement of the limiting 



angle, 



lie n ry 



COS. X = ^ , X = 2G ; 



but that the faces of entrance and emergence may be cut perpen- 

 dicularly to the mean direction of the rays, the angle of refraction 



of the extreme rays is ^ and the angle of incidence y, so that 



The field (maximum in these conditions) is 4i° ; the instrument 

 will take this in completely ^Yithout rotation if the face attached to 

 the spar is the third of the other, the aperture for the eye being 

 near its edge. The angle adjacent to the sjDar is 90° — 13'^ = 77°. 



To regulate the intensity of the two images, a polarizer may 

 be interposed in the path of the most luminous rays, such an 

 apparatus, for example, as M, Cornu's made of the materials above 

 mentioned. 



No drawing accompanies M. Pellerin's paper. He adds that a 

 camera lucida of the same description may be made for vertical 

 Microscopes by replacing the quadrangular prism by a parallelo- 

 piped with an angle of 77°. 



(3) The third arrangement is that of Mr. James Swift, shown 

 in Fig, 5, and can be used at any inclination of the Microscope. 



1'he principle of the instrument, as described by Mr. Swift, is 

 that the image of the pencil and paper is received by a prism 

 (enclosed in the box which projects on the 

 left-hand side of the figure), by which it is 

 reflected to a piece of neutral-tint glass placed 

 at an angle of 45° over the centre of the upper 

 lens of the eye-piece. The neutral-tint glass 

 allows the image of the object in the Micro- 

 scope to be distinctly seen, while that of the 

 pencil and paper is at the same time visible on 

 its first surface; no second image occurs by 

 reflection from the back surface, omng to the tint of the glass. 



A second disk of neutral-tint glass can be interposed when the 

 light requires to be subdued to show the point of the pencil distinctly. 

 It will be seen that in principle the instrument is an adaptation of 

 Nachet's well-known form. 



(4) The fourth form is that of Dr. Eussell, which will be 



Fig. 5. 



