148 OPTICAL PROJECTION 



other for a single one, which latter, however, even with the 

 lime-light, does not afford sufficient illumination to exhibit a 

 carte, de visite more than three or four feet in diameter. 

 Taking the double arrangement in fig. 78, the two lanterns 

 L L are turned away from the screen, the objectives removed, 

 and their two nozzles fixed to holes cut to fit them in the box 

 B. To the back of this box the two grooved doors D D are 

 attached by a common hinge, so that when one door is closed 

 the other is outside for the attachment of a new slide. One 

 of the lantern objectives is fitted on the front of the box at 

 A, and the carte or other object is shown by the light reflected 

 from its surface, as shown by the dotted lines. The single 

 instrument M shown in fig. 77 acts on precisely the same 

 principle, but with only half the illumination. 



A new pattern has been lately introduced by several 

 English and Continental opticians. The body of the lantern 

 itself is so constructed that when the objective is removed, a 

 reflector can be introduced hi front of the condenser, which 

 throws the light back to a place above and behind the con- 

 denser, where there is a fitting to receive objects or slides. 

 In front of this stage for opaque slides is another opening to 

 which the objective can be attached. There are thus two 

 positions in which the objective may be used, the ordinary 

 one, and one above for opaque slides, an oblique reflector 

 being always necessary when these are shown. 



Coins and medals show very well with the opaque lantern. 



81. Microscopic Attachment. The ordinary form of pro- 

 jecting lantern is itself a microscope, since it projects upon 

 the screen a magnified image generally about 50 diameters 

 of the slide or object. There is no difference whatever in 

 principle between doing this, with a lens of say 6 inches focus, 

 and producing an image of 2,000 diameters with a lens of ^- 

 inch focus. The practical difficulties are connected with that 

 management of the rays discussed in Chapter I ; and these 

 are very great as we approach the higher powers. Up to a 



