446 ON APEETUEE AND DEEINITION OE MICEOSCOPE OBJECT GLASS. 



through the dioptric media of the eye.* But secondly, the table shows 

 that increase of aperture is not so efficient in resolving the extreme 

 minute lines or spaces when the aperture is increased beyondllO® as 

 when it is being raised from a comparatively low figure up to this de- 

 gree. I^either is ** variation of defining power ivith the cliord of the angle 

 of aperture " (J. Hoggf ) to [be understood as one of progressive 

 excellence due to increase of aperture, nor, indeed, is this use of 

 ihQ ievm. defining power ''in absolute agreement with " any teach- 

 ing of Helmholtz concerning definition, or with any theory of 

 difi'raction phenomena which interfere with the vision of closely 

 ruled lines. This appears even from Mr. Sorby's own table 

 calculated from the formula given by Helmholtz. For, if the 

 figures in this table (copied from the Monthly Microscopic Journal, 

 for March), 



Eed End of Spectrum . . . 



Mean Kays 



Blue End 



ISO'* 



1 



74000 



92000 



120000 



* This difference in discrimination of colour by different parts of the 

 retina is a normal one, but is not evident when white light is used because 

 the middle rays greatly predominate. The difference reaches its climax in 

 persons who are colour blind, the commonest form of which is red blindness, 

 when red colour (and the red end of the spectrum) cannot be seen at all. 

 The hypothesis of Dr. Young is accepted by Helmholtz, viz., that one kind 

 of nerve when excited by the longest undulations induces a sensation of red 

 light, a second nerve excited by medium undulations induces green light, and 

 a third nerve excited by the shortest undulations conveys impression of \dolet. 

 The latest anatomical researches tend to show that a triple strand of this kind 

 forms the rather thick nerve which connects the cones with the ganglionic 

 layers of the retma. 



t See Mr. Sorby's address (loc. cit.), page 110, line 12. 



