ON INSTRUMENTS AND THEIE USE. 21 



If a set of concentric circles be observed with one 

 eye, they are seldom all distinct at the same time, 

 and there is produced a kind of Maltese-cross effect, 

 not perceivable, perhaps, in many instances with 

 large circles, but noticeable when drawn to such 

 a size that the outer one is about two inches in 

 diameter. 



This defect is called astigmatism, and known to 

 oculists as a common cause of headaches. Spasm 

 of the focussing apparatus may derange the spheri- 

 city of the eye, and so affect vision. Strained vision 

 is very subject to this. On the other hand, the 

 same apparatus may be paralysed, and ordinary 

 vision deficient, whilst the focussing of the micro- 

 scope might possibly correct it. 



Astigmatism has injuriously affected painters ; 

 Turner, for instance, whose later pictures are dis- 

 covered to be slightly distorted, in consequence of 

 the power of accommodation or self -correction 

 having been lost through age. 



In microscopic drawing, as with the camera 

 lucida, the perspective may be misrepresented, in 

 consequence of astigmatism, and thus endless dis- 

 putes may arise even among the most careful 

 observers. 



We have now to deal with errors of refrangibility, 

 and it will probably have been assumed that the eye 

 apparatus is entirely corrected for colour. This is 

 not the case, however, except when an object is in 

 exact focus, and the reason that the error due to 

 refrangibility remains practically unnoticed is that 

 the distance between the focal point of the red and 

 violet rays is extremely small. The error due to 

 refrangibility may be noticed by means of the con- 



