46 



THEORY OF THE MICROSCOPE. 



FIG. 10. 



or within the microscope-tube. The action of a right-angled 

 prism employed for this purpose is exhibited in Fig. 10. It pro- 

 ^ z duces a lateral inversion, but none in a direction at 

 right angles to the plane of the paper ; to complete 

 the inversion, a second prism at right angles to the 

 first must therefore be added. It is evident that such 

 a combination can be replaced by a single prism, so 

 constructed that two equivalent total reflexions take 

 place within it. Both methods were variously em- 

 ployed by Chevalier, Nachet, Amici, and others. The 

 most practical contrivance is the erecting prism (prisme 

 redresseur) now supplied by Nachet, enclosed in a box, 

 which may be attached or removed at will. This 

 prism is shown in Fig. 11. Its under face is at right 

 angles to the axis of the Microscope, and forms an 

 angle of 58 with the upper face. The inclination of 

 these faces is calculated to bend the optic axis to an 

 angle of 30 from the horizon, which is convenient 

 for working. The faces ab c d and beef, which meet in the edge 

 c b, are the reflecting ones ; they form an angle of 81J, so that 

 the planes of reflexion of rays incident from beneath 

 cut one another, as calculation proves, at right angles. 

 The other faces of the prism are without influence 

 upon its optical action. 



If we were willing to forego the convenience of 

 removing the prism at will, the under face might 

 with advantage be made convex, to replace the 

 eye-lens a construction which Nachet adopted in his older erecting 

 Microscopes with two prisms. 1 The inversion of the image would 

 be accomplished in this case without loss of light. 

 The improved erecting prism (prisme redresseur 

 />< /-f't-Honne, Fig. 12), which Nachet has introduced 

 in his latest catalogue, is combined with a special 

 form of eye-piece for the enlargement of the 

 field of view. The new erecting eye-piece of 

 Hartnack is similarly constructed; according to 



S*s ,._., Frey it consists of a complex prism above the 



Fir,. 12. eye-lens, and gives a very bright though some- 

 what small field of view. 



" Mikr." 2nd ed. iii. p. 228. 



