NACHET S ERECTING PRISM. 



91 



by the formation of the first image much nearer the objec- 

 tive, that, if a lens 



of 2-3rds of an inch FlG - 47 - 



focus be employed, an 

 object of the diameter 

 of lg inch can be 

 taken in, and enlarged 

 to no more than 4 

 diameters ; whilst, on 

 the other hand, when 

 the tube is drawn-out 

 4 \ inches, the object 

 is enlarged 100 diam- 

 eters. Of course every 

 intermediate range can 

 be obtained by draw- 

 ing out the tube more 

 or less ; and the facility with which this can be accomplished, 

 especially when the Draw-tube is furnished with a rack -and -pinion 

 movement (as in Messrs. Smith and Beck's Compound Dissecting 

 Microscope), renders such an instrument very useful in various 

 kinds of research. 



65. Nachet's Erecting Prism. — An extremely ingenious arrange- 

 ment has been made by MM. Nachet, on the basis of an idea first 

 carried into practice by Prof. Amici, by which the inverted image 

 given by the Compound Microscope is erected by a single rec- 

 tangular Prism placed over the Eye-piece. The mode in which this 

 prism is fitted up is shown in Fig. 48 ; the rationale of its action 

 is explained by the diagram Fig. 47. The Prism is interposed be- 

 tween the two lenses of the eye-piece, and has somewhat the form of 

 a double wedge, with two pentagonal sides, abode, and abhgf, 

 which meet each other along the common edge a b, and two facets, 

 J) E p G, and cdgh, which meet along the common edge d g, the edges 

 A b and d G being perpendicular 



to each other. The rays emerging 

 from the Field-glass enter this 

 prism by its lower surface, and 

 are reflected at 1 upon the face 

 abhgf, from which they are 

 again reflected upon the lower 

 surface at the point k, and 

 thence to the point L upon the 

 vertical face cdgh, and lastly 

 at the point m upon the other 

 vertical face defg; from which 

 the image, normally and com- 

 pletely erected, is again sent 

 back, to issue by the superior 



Fig. 48. 



