344 



Transactions of the Society. 



The fine centring is performed as follows : — The vertical portion 

 of the indices takes the form of a trident the tips of which are 

 vertically in line, as shown at A in fig. 4. If the indices are now 

 adjusted so that the upper prong is in contact with the edge 

 of the cone of illumination, then if the indices lie in a true 

 diameter the lower prong will also be just in contact, see fig. 4 

 at B. If contact is not perfect and the index is set apparently at 

 too high a reading, as shown at C, the apertometer plate should be 

 shifted further back and vice versa. Adjustment should be made 

 until the upper and lower prongs appear to meet the edge at the 

 same instant, as shown at B in fig. 4. (At D is shown a better, 

 but somewhat more complicated form of index.) 



A. 



B. 



« 



COMPLtTC ADJWTMftWT. 



D. 



I WCOnPLETE A t'-'"^'^'^^^"'' 



CLASS piSC iNoex 



Fig. 4. — Diagrams to show new form of index which facilitates the 

 setting of the plate at right angles to the optical axis. 



At A the new form of trident index is shown. 



At B the index is shown in relationship to the aperture, the angle 



between the plate and the optic axis being a right angle. 

 At C the index is shown in relationship to the aperture, the angle 



being greater than a right angle. 

 At D an alternative and better type of index is shown. It is used in 



a similar manner to the one shown at A. 



