Manchester Memoirs, Vol. Iv. (191 1), No. %\. 7 



the lens, when passing through it at a distance a scale 

 divisions from the centre. It is, therefore, approximately 

 measured by the ratio EC/FC or by a scale divisions 

 divided by/" metres, where /metres is the focal length of 

 the lens. 



Thus we have : — 



S scale divisions a scale divisions 

 X metres "" /metres 

 or S/a = x\f. 



If we now agree to make the radius a of the aperture 

 equal to i/,t scale divisions, then : 

 S=i//=I) 

 where D is the power of the lens in dioptrics. 



The number of scale divisions from the zero division 

 M to the centre N now becomes i/.t'+i/;'. 



The choice of a scale to be used in this method will 

 depend upon the range of focal powers to be tested, and 

 the radius of the aperture must not be too great for the 

 size of lens to be used. An ordinary millimetre scale can 

 be used if the distance x-\-y is not too great. 



It is an advantage to use a scale made in the form of 

 concentric circles with N as centre, so that the power of 

 the lens in different planes can be read off without 

 rotating the scale. 



This was the method adopted by Dr. Guilloz, Professor 

 in the Faculty of Medicine at Nancy, and was described 

 in 1895 to the Congres pour I'avancement des Sciences at 

 Bordeaux. The type of focometer required for the purpose 

 was made by Pellin (the successor of Duboscq). We have 

 simplified the instrument and modified it so that it can be 

 cheaply constructed. We think that in the new form it 

 will be of considerable service in optical measurements, 

 and will help in making students familiar with the use of 

 the dioptrie. 



