Examination of Photographic Lenses at Kew. 



447 



Objections have been raised to the use here made of the term 

 astigmatism, when it is intended to mean the effect of spherical 

 aberration on oblique rays ; it has been proposed to limit the use of 

 the word so as merely to signify cylindricity in the lenses, such as 

 might be produced by turning them in a lathe with elliptical motion. 

 Whatever may be the theoretical value of this objection, we fear that 

 the use of the term has been so thoroughly incorporated into the 

 photographic vocabulary, both in England and abroad, that it would 

 now be impossible to substitute another expression in its place. 



17. Illumination of the Field. The figures indicate the relative intensity 

 at different parts of the plate. 



With C.I. Stop No. 

 At the centre 100 



At inches from the centre 

 At inches from the centre 



With C.I. Stop No. . 



Ditto 100 



Ditto , 



Ditto . . 



The intensity of illumination of the field is always greatest near 

 the axis of the lens, and falls off more or less rapidly towards the 

 edges of the plate. The lens should therefore be examined with the 

 view of ascertaining if this inequality of illumination is greater than 

 that which experience shows must be tolerated under given circum- 

 stances. The apparatus employed for conducting this test is shown 

 in fig. 23, the method being devised by Captain Abney. There is a 



FIG. 23. 



fixed lamp, L, the position of which is not changed during the ob- 

 servations ; F represents a paper screen, placed in that position in 

 order to give a practically uniform source of light ; O is the lens, 

 which is fixed in a frame, not shown in the sketch, revolving about 

 the pivot N ; by means of a suitable adjustment, this axis, 1ST, is made 

 to pass through the nodal point of emergence of the lens. At S there 



