MancJiester Memoirs, Vol. Iv. (191 1), No. %y. 



XXI. Dioptriemeters. 

 By Prof. VV. W. H ALDAN E Gee 



AND 



Arthur Adamson. 



Read April 4II1, igii. Received for publication, April 2^th, rgii. 



In 1872 Monoyer* proposed the term dioptrie as the 

 unit of focal power. It represents the focal power of a 

 lens one metre in focal length, and since the focal powers 

 are the reciprocals of the focal lengths, a lens of two 

 metres focal length will be half of a dioptrie, a lens of half 

 a metre focal length will be two dioptrics, and so on. 

 This unit proved of so great a convenience in practical 

 optics that it was finally adopted in 1875 by the Inter- 

 national Medical Congress at Brussels, and by the Inter- 

 national Congress of Ophthalmology at Heidelberg. 

 Unfortunately, a uniformity of spelling has not been 

 followed, so that we find the unit is also given the names 

 of dioptry, dioptre, and diopter. The term dioptric for the 

 unit is also used by A. Bruce.f Since some of these 

 varieties have other meanings, it would be desirable if the 

 original spelling were followed, so as to conform to French 

 and German usage. S. P. Thompson has proposed to 

 extend the use of the dioptrie for the measurement of 

 curved surfaces in general. 



* " Annales d'Oculistiqiie," vol. 68, p. iii. 

 t " Encyl. Biit.," vol. 22, p. 375, 1S87. 



June 1st, igii. 



