CH. X] DRAWING WITH THE MAGIC LANTERN 331 



reduction of the picture can be determined by photographing a 

 metric or other measure (fig. 178) on the same plate with the 

 object or piece of apparatus. 



467. Size of condenser necessary for making drawings. 



When lantern slides, or negatives made on lantern-slide plates or 

 other plates of that size are used, the condenser of any magic lan- 

 tern will answer. Sometimes, however, it is desired to make dia- 

 grams or drawings from negatives of larger size. There are two 

 ways of accomplishing this: 



(1) A lantern slide can be made from the large negative by the 

 aid of a photographic objective as described in Ch. VIII, 329. 

 This can then be used in the ordirary lantern. 



(2) If the large negative is to be used direct, then the condenser 

 of the magic lantern must be of sufficient size to illuminate the 

 negative. That is, the condenser must have a diameter a little 

 greater than the diagonal of the negative to be illuminated and 

 drawn (see fig. 114). 



468. Drawing on a horizontal surface by the aid of the magic 

 lantern. This is easily accomplished by using a 45 degree mirror 

 or a prism beyond the objective (fig. 192). 



One must be careful to put the negative or lantern slide in the 

 carrier in such a way as to give an erect image ( 512). 



If the negative or lantern slide or other object is too dense, so 

 that the light is relatively dim, the image will be duplicated when a 

 mirror silvered on the back is used, therefore, one must use a prism 

 or a mirror silvered on the face for these dark objects. For very 

 transparent objects the image appears single even with a mirror 

 silvered on the back, the silver image being so much brighter than 

 the glass image that the latter does not show. 



One can use the magic lantern and separate table with a 45 degree 

 mirror (fig. 182) or the mirror can be fastened to the projection 

 table as in Dr. Riley's device (fig. 184) or the mirror may be close 

 to the objective, and the adjustable drawing shelf used (fig. 

 183). 



