77 = 



No. 124 or 125. *Screw Micrometer Ocular. This apparatus is for per- 

 forming very exact measurements. It consists of a RAMSDEN ocular of about 

 20 mm ( 3 / 4 in.) focal length corresponding magnification = = 12 diameters or 

 a suitable compensating ocular and a very carefully constructed micrometer with 

 divided drum. (Fig. 38.) The glass plate, with a crossed as well as a double 

 line, is moved across the field by means of the micrometer screw. Each interval 

 of the divided drum corresponds to a movement of 0.01 mm of the point marked. 

 Complete revolutions of the drum are counted by means of a numbered scale 

 seen in the field. The measurement can be extended over a tract of 4 mm of 

 the objective image. 



The absolute values corresponding to the divisions of the drum should be 

 determined by the aid of a stage-micrometer for each separate objective. 



Fig. 38. Screw Micrometer Ocular, No. 124. 



Sectional elevation and plan view. (*/ 3 Full Size.)' 



Screw Micrometer Ocular with RAMSDEX ocular for 

 achromatic objectives, in case 



Screw Micrometer Ocular with compensating ocular 6 

 for apochromatic objectives, in case 



Marks Codeword 

 90 Peal 

 105. Pebetero 



Qa.it /&iitt, Sena. 



