4:70 Dr. Q. V. Drysdale on the 



a lens or mirror, and is termed a Dioptre * This unit is a 

 curvature corresponding to a radius of one metre ; and we 

 consequently say that a lens has a convergence' of one 

 dioptre when it has a focal length of one metre, or that a 

 wave-front or surface has a curvature of one dioptre when it 

 has a radius of curvature of one metre. The other unit 

 adopted is one of angle or deviation and is termed the Prism 

 Dioptre : it is defined as a deviation of one centimetre on a 

 tangent line at a distance of one metre, and one prism dioptre 

 therefore corresponds approximately to '01 of a radian. The 

 prism dioptre and the radian thus serve as two units of 

 angular measurement, and the former is very conveniently 

 related to the dioptre, as the deviation in prism dioptres 

 produced by any thin lens at any zone is simply obtained by 

 multiplying the convergence of the lens in dioptres by the 

 radius of the zone in centimetres. 



In the prism dioptre and the radian we have two units of 

 angular measurement which cover most requirements ; but 

 the curvature dioptre, although an exceedingly convenient 

 unit for many purposes, is inconvenient to deal with when 

 we are concerned with microscope lenses and others of short 

 focal length. The writer has therefore recently proposed f 

 the adoption of multiples and sub-multiples of this unit, using 

 the ordinary prefixes as follows : — 



Radius of curvature Curvature. 



Kilometre Millidioptre. 



Metre Dioptre. 



Centimetre Hectodioptre. 



Millimetre Kilodioptre. 



The great advantages of such a system is, that to any 

 particular length measurement fchere is a correspondino- 

 curvature measurement, which enables one to have a physical 

 idea of the magnitudes involved and avoids the use of un- 

 wieldy fractions which frequently occur otherwise. The 

 range of curvatures may be very great, the radius varying 

 from a wave-length to infinity. 



Notation. — The writer has also found it convenient to 

 adopt a standard notation in optical work, using small letters 

 lor the distances, radii of curvature, focal lengths, and 



* Variously spelt, Dioptrie, Dioptre, and Diopter. The author has 

 hitherto preferred the spelling Dioptre, harmonizing with metre, but 

 modern usage favours meter and diopter. It appears, however, that 

 Dioptrie was proposed by Monoyer in 1872, and adopted bv the Brussels 

 International Congress in 1875. 



t "On some Points in the Design of Optical Instruments." Proc 

 Optical Soc, December 18th, 1902. 



