796 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



coincidence of the point of the scale with that of the micrometer is 

 obtained by the screws r and s acting on the plate u u, which is not 

 rigidly fixed to the cylinder a, but slightly movable. 



Geneva Co.'s Microscope Callipers. — In the instrument, fig. 126, 

 (made by the Societe Genevoise pour la construction d'Instruments 

 de Physique), a compound Microscope is made use of for measuring 

 minute thicknesses such as cover-glass, &c. It consists essentially of 

 a lever at one end of which are the jaws for holding the object to 



Fig. 126. 



be measured (shown in the figure with a piece of glass between 

 them), and the movement of which is amplified twelve times. At 

 the other end the lever carries a glass plate ruled with 120 divisions, 

 which is observed through a Microscope having a fixed micro- 

 meter in the eye-piece with 30 divisions. The jaws are opened by 

 the milled head on the box, and the extent of movement is indi- 

 cated by a scale with 120 divisions (corresponding to the glass plate), 

 which passes under the aperture seen at the top of the box. By the 

 eye-piece micrometer the principal divisions may be further sub- 

 divided. When open the jaws are 3 mm. apart ; each of the principal 

 divisions represents therefore 1/40 mm., and the subdivisions 

 1/1200 mm. The mirror illuminates the divisions of the glass plate. 



