340 



Transactions of the Society. 



Section 3. — Ereok in Apertometey due to the Incorrect 



Setting of the Indices. 



Probably the most accurate of optical methods of measurement 

 is that in which an index is caused to coincide with the centre of 

 the image to be measured (^). 



But the accuracy is enormously decreased if the field on one 

 side of the image is invisible to the observer, because the measure- 

 ments are then affected both by his visual acuity and his judgment. 

 If the rays from the objective be traced through the apertometer it 



B. 



Tips invisible 

 reaoini^ too m ish 



Tips visisuc 



fltAD\N(j^ TOO LOW 



c. 



D. 



InTERVAL INVlSii^?; 

 REAOII4Q TOO H (QH 



Imterval visible 



READINQ -rCO LOW. 



Fig. 1. — Diagrams to show how error is caused by the restriction 

 of the visible field to the aperture of the objective. 



At A the indices have been adjusted till the tips are judged to be 



invisible. The reading is now too high. 

 At B the indices are still visible, and therefore the reading is too low. 

 At C similar conditions to A are not met with. 

 At D the conditions are similar in their effects to those at B. 



