THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



Vol. XII, DECEMBER, 1891. No. 12. 



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 can Mo7ithiy Microscopical Journal, Washington, D. C. 



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 Triibtier &^ Co., 57 Ltidgate Hill, Lo7idon, or to Mr. W. P. Collins, 757 Great 

 Portland street, Londoti, accompanied by the yearly price of five shillings. 



The Grapho-Prism and its Use. 



By FREDERICK GAERTNER, M. D., 



PITTSBURGH, PA. 



[With Frontispiece.] 



To a practical microscopist who is not also a skilled artist perhaps 

 nothing is more important among microscopical accessories than the 

 camera lucicla or grapho-prism. This prism is an instrument for sketch- 

 ing objects with the point of a pencil upon a piece of paper laid beside 

 the microscope. By its use a high degree of accuracy maybe attained. 

 Perhaps the simplest and most successful drawing prism is that of Ziess 

 (Fig. i) , which is followed by that of Nachet (Fig. 4) , Abbe (Fig. 5) , 

 and Oberhauser (Fig. 6). Nobert's and many others work upon the 

 same principles. 



The following is an explanation of the principle of this drawing ap- 

 paratus : If the glass plate _^/ (Fig- 2) stands at an angle of 45 degrees 

 (45°) with the. axis of the eye, the rays from the objects (which on their 

 part also forms an angle of 45° with the glass plate) ai'e reflected, and 

 the picture of the object is seen in a position that also forms a right angle 

 with that of the object. If »^ (Fig. 2) is the cylinder of the microscope 

 and pp the piece of paper, in this case the eye vt^ill see upon the paper 

 at o the pictui'e which is projected by the transparent condition of the 

 glass plate _^/. In this case we say that the picture is projected ; but if 

 we place a prism p (Fig. 3) upon the same level with the glass plate 

 gl (Fig. 3), and o is the object under the microscope standing in a ver- 

 tical position /;z, the glass gl forming an angle of 45° with the axis of 

 the eye and standing upright over the ocular, we will then see the pic- 

 ture at o upon pp. Meanwhile the projected picture of the object may 

 also be seen in the horizon. Upon this basis rests the above-mentioned 

 drawing prism of which Nachet's (Fig. 4) is the one most commonly 

 in use in Europe after Ziess's. In this apparatus a prism is employed in 

 place of the glass plate, while a second grapho-prism moves upon its own 



Copyright, i8gr, by C. W. Smiley. 



