THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL, 



Vol. XII. DECEM6EB, 1891. No. 12. 



All communications for this Journal, whether relating to business or to editorial 

 matters, and all books, pamphlets, exchanges, etc., should be addressed to Ameri- 

 can Monthly Microscopical Journal, Washington, D. C. 



European subscriptions jnay be sent directly to the above address accompanied 

 by International Postal Order for $1.13 per annum, or they may be sent to Messrs. 

 Triibner &> Co., j/ Ludgate Hill, London, or to Mr. W. P. Collins, i^j Great 

 Portland street, London, accompanied by the yearly price oj 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 lucida 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 may be attained. 

 Perhaps the simplest and most successful drawing prism is thatof 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 ^'Z (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) are reflected, and 

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

 with that of the object. If w (Fig. a) is the cylinder of the microscope 

 and pp the piece of paper, in this case the eye will see upon the paper 

 at d the picture 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 is the object under the microscope standing in a ver- 

 tical position w, 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, 1891, by C. W. Smiley. 



