The telescope axis is perforated at both sides. Into one side 

 is inserted a small lens mount by which the rays are concentrated 

 upon a very small 45 reflector, as shown in the illustration. The 

 long slender mounting for the reflector is attached to a plug which 

 is inserted into the opposite axis perforation as indicated. If the 

 illumination is so brilliant as to obscure the rest of the field, it can 

 be subdued by slightly turning the plug which sustains the reflector 

 mounting. In this respect it is superior to the polished sphere 

 which has also been used for this purpose. 



The Micrometer Microscope 



Micrometer microscopes are usually employed in pairs on the 

 larger and more finely divided circles of alt-azimuths in place of 



the vernier scales commonly used 

 for smaller and less accurate in- 

 struments. 



This appliance consists of a 

 compound microscope with a 

 movable twin filament in the 

 focal plane that is intended for the 

 measurement of smaller spacings 

 than are indicated on thegraduated 

 circle. According to one of the 

 fundamental theories of micros- 

 copy, the objective of the micro- 

 scope produces an image of the 

 spaces and numbers at the microm- 

 eter diaphragm, the size of which 

 depends upon its focal length and 

 its distance from the graduated 

 circle. 



The focal length of the ob- 



f. 83 jective and the pitch of the 



micrometer screw are fixed conditions, but by carefully regulating the 

 distance between the objective and the graduations, an image of the 

 spaces can be formed that will exactly equal the pitch of the thread 

 or some multiple thereof. If the limb is divided into l /i^ spaces, 

 for instance as shown in Fig. 84, we may utilize theconditions given 

 as above so that it will take five revolutions of the screw to draw the 

 twin wires over one of the image spaces. This i. 

 ient to assign ^fc X l /b, or 2', to each revolution of the drum, and if 

 the drum is sub-divided into 60 equal spaces we can allov, 

 or 2", for each division of the drum. In this case both drum heads 

 are numbered 0, 10, 20, 0, 10, 20. The reason for this is that the 



