298 PROJECTION MICROSCOPES ON THE MARKET [Cn. IX 



W 8 The wire to the left lamp, lower carbon. 



W 6, W 7 Wire including the rheostat, passing to the upper carbon of the 

 left arc lamp. 



L Rheostat Rheostat for the left lamp. 



r 3, r 4 The binding posts of the left rheostat. 



L Lamp The left arc lamp. 



F Feeding mechanism for the carbons. 



d Clamp for fixing the arc lamp in any vertical position on its standard. 



5 s Set screws for the carbons. 



// C Horizontal or upper carbon. 



V C Vertical or lower carbon. 



R Rheostat Rheostat for the right arc lamp. 



r i, r 2 Binding posts for the rheostat. 



W 3 W 4 Wire from the switch through the right rheostat to the upper 

 carbon of the right arc lamp. 



W 5 Wire to the lower carbon of the right lamp. 



R Lamp The right lamp. It is exactly like the left one. 



L Lamp, R Lamp The arc lamps for the two projectors. 



Condensers The triple-lens condensers with water-ceils for the two pro- 

 jectors. 



Axis, Axis, Axis, Axis Principal optic axis in the two projectors. 



P Objective The projection objective at the left. 



Microscope The projection microscope at the right. 



Screen Image, Screen Image The images formed on the screen by the two> 

 instruments. 



NOTE. In using these projectors it is only necessary to turn the switch 

 handle over to the one desired and that lamp can be lighted. One can turn 

 from one to the other at will. 



A more economical arrangement would be to have a single rheostat inserted 

 along either Wi or W2 before reaching the knife switch, then the single rheostat 

 would serve for both lanterns. 



With the two rheostats, as here shown, both lanterns could be run at the 

 same time if there were two switch handles and double blades hinged at the 

 center (II i, H 2). 



The projection microscopes in the open market rarely possess 

 anything like this range. Very few will project an object as great 

 as 25 mm. in diameter. 



It seems to the writers of this book that the makers have unduly 

 limited the range of their apparatus by a too rigid insistence on the 

 use of substagc condensers and projection oculars, and also by the 

 effort to make combined apparatus. Combination always means 

 compromise and more or less loss of individual efficiency. 



It is certain, too, that most of them have not fully appreciated 

 the necessity for dull black surfaces. The bright finish is probably 

 to please the eye when the apparatus is not in operation. It 

 certainly is not good for the eyes when the apparatus is in opera- 

 tion. 



