RemarJcs on Prof. Ahbe's Note, &c. By Br. E. Giltaij. 967 



an object of 1 mm. will appear under a visual angle whose tangent 

 is 2-4. 



I have had the opportunity of observing in another way that 

 the expression of Prof. Abbe is not so well adapted to give a 

 practical idea of the power of lenses. 



For some time I have been accustomed to distinguish lenses in 

 the way proposed by Prof, xibbe. In ophthalmology, lenses are, at 

 least on the Continent, somewhat generally denominated by a 

 decimal fraction, equal to a vulgar fraction whose numerator is 1 

 and whose denominator is the focal distance expressed in metres. 

 It is not difficult to see that this manner of denoting lenses 

 exactly coincides with Prof. Abbe's way of expressing the power of 



optical systems (for tan u =. - , Abbe, 1. c). 



Although this denomination is very convenient for finding, by 

 the simple addition or subtraction of two lens-numbers, the value of 

 the lens, which is equivalent to the added or subtracted action of 

 the component lenses ; and although, therefore, this method is in 

 ophthalmology very practical, yet 1 never could see that the num- 

 bers of spectacle-glasses give so good an idea of the lenses in 

 themselves as the amplification numbers, or even as the values of 

 the focal distances. Perhaps this is a subjective peculiarity of 

 mine, but in that case my other remarks remain unaltered. I think, 

 therefore, that if we have to choose between the old and the new 

 mode of expressing power, we may give the preference to the 

 former, if only the proper ideas which can be attached to its 

 meaning are actually attached to it. 



3 R 2 



