824 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and also a removable collar carrying the tube 0, which, is supported by a 

 removable pillar shov^n in fig. 227. Inside the tube C is a second tube 

 made to work back and forth very easily, and carrying at its lower end a 

 right-angle prism set for total reflection. This tube is of such a length as 

 to give, when in position for receiving the image from the objective through 

 the prism, a length of 10 in. measured along the optical axis. The eye- 

 piece in the outer end has cross-hairs set in the diaphragm, so adjusted 

 in relation to the prism at the other end as to correspond with cross lines 

 on the ground glass of the camera screen. The tube C, therefore, gives 



Fig. 229. 



the opportunity to examine the object under the conditions of microscopic 

 vision, and with the cross-hairs in the eye-piece farther enables the 

 operator to exactly centre the object on the screen. 



F is an adjustable tube carrying within it a second tube, which may be 

 slid back and forth. This interior sliding tube has an adapter at the front 

 end, into which an amplifier may be screwed, and the whole racked back 

 and forth by the pinion E, which also carries between the thumb-screw 

 and the body a pulley over which a band can be passed for working the 

 amplifier from the rear of the camera screen. 



This inner tube also has a graduation on the side, in order to facilitate 

 recording the proper position of the amplifier for various extensions of the 

 screen. 



In working with high powers where it is desirable to use the amplifier, 

 the objective is set to normal working distance by observing the object 

 through the tube C. The operator then, from the rear of the camera, by 

 use of the band over the pulley at E, racks the amplifier to such a position 

 as to give a sharp and distinct image on the ground glass, the objective in 

 the meantime remaining undisturbed. It is of course understood that after 

 having adjusted the objective to normal working distance the inner tube at 

 C, carrying the prism and eye-piece, has been sufficiently withdrawn to 

 allow the rays of light to pass unobstructed to the camera screen. This 

 gives us almost instantly the conditions which have been shown above to 

 be necessary for production of the highest results, and with this apparatus 

 the most difiicult tests are easily photographed. 



When working with low powers the amplifier is not essential for the 

 production of sharp images, and the tube C and nose-piece B are removed 

 by simply slipping off the collar from the nose-piece, and unscrewing the 

 nose-piece from the body, an operation which may be performed in a 

 moment. Fig. 229 shows these parts when detached. 



After removing B and C, the inner tube F is drawn forward so that the 

 front end of it occupies approximately the position of B when in place, and 

 the objective is screwed into the adapter in the end of said tube F, which in 

 high-power work carries the amplifier. 



D is a second tube back of F, with prism and eye-piece with cross-wires, 

 precisely as in C. With this tulDe the object is examined and centered on 

 the ground glass, as above described, for work with C. After such centering 



