324 SUMMARY OF CURRENT RESEARCHES RELATING TO 



(fig. 46) is a metallic diaphragm surmounted by three or four springs 

 slightly divergent, as in the indicator diaphragm. But the opening is 

 square or rectangular and carries a thread on its upper face, set so as to 

 be nearer to one of the parallel sides than the other. M. Malassez 

 considers that this quadrangular opening affords a surer means of 

 searching a field than a circular one, and the unequal division renders 

 it easier to keep a check on computation. 



In the second model (fig. 47) the thread is movable, and, as in the 

 indicator diaphragm, a slight inclination of the ocular causes appear- 



Fig. 45. 



Fig. 46. 



Fig. 47. 



As will be understood from the 

 folds back through an angle of 



ance or disappearance of the thread, 

 figure, the framework of the thread 

 180°. 



(iii.) Diaphragm with Movable Glass Slip. — In this case the dia- 

 phragm is intended to receive any kind of glass micrometer, squared or 

 otherwise. M. Malassez has designed two types. 



The first type (fig. 48) has a general resemblance to the preceding ; 

 but the three springs are set at unequal distances around the periphery 



Fig. 50. 



Fig. 48. 



Fig. 49. 



in order to leave a wide space between two for the insertion of the 

 micrometer slip. The periphery also is made with a small vertical 

 flange, except at the above gap. The slip is kept in position by means 

 of a semicircular spring, whose two extremities gently press on it. It 

 is advisable to set the slip with the ruled surface downwards. The 

 arrangement works excellently when the diaphragm is always used by 

 the same person ; but if persons of different visual power have to use 

 the diaphragm, or if, for any other reason, the focus has to be varied, 

 further construction is necessary. In one method M. Malassez screws 



