136 



THE GRAPHO-PRISM AND THE TECHNIQUE 



parent condition of the glass plate, gl. In this case we say that 

 the picture is projected ; but if we place a prism,/'. (Fig. 20), upon 



/' 



-/' 



r-^ 



Fig. 19. Fig. 20. 



the same level with the glass plate, gl. (Fig. 3), and 0. is the object 

 under the microscope, ///., standing in a vertical position — 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 picture at 0^ pro- 

 jected upon the paper, //. Upon this principle is also constructed 

 the drawing-prism of Nachet (Fig. 21). In this apparatus a prism 

 is employed in place of the glass plate, while a second prism 

 moves upon its own axis so as to bring the reflecting surfaces at 

 different angles. 



The use of a drawing-prism is obvious, as soon as the drawing- 

 prism has been placed upon the ocular and adjusted by means of 

 a ring or other attachment. The drawing of a picture, whether of 

 a microscopic or macroscopic nature, nowaday is considered a scien- 

 tific achievement, especially so when the sketch is made by " free- 

 hand." 



In the drawing of a macroscopic object (gross appearance) the 

 principal considerations are the direct measurements, and these 

 can easily be obtained by the use of circles, lines, rules, strings, 

 and other metrical appliances on the one hand, and on the other 

 by the use of a camera, photographic apparatus, magic lanterns, 

 etc. etc. Of course (in this case) the object aimed at is, first, to 



