56 Dr Goring on Monochromatic Light, #c. 



fruitful source of deception it must prove as to the achroma- 

 tism of these instruments ! 



It is no wonder that we should have so many kinds of monochro- 

 matic lamps and light, when a common candle will answer just 

 as well. As prisms and lenses have a great analogy to each other 

 in their operation, I then experimented on the dispersed solar light 

 with them, and it always appeared to me that when I could suc- 

 ceed in getting the light from the diaphragm thrown perfectly 

 direct through a prism, no sensible secondary dispersion was 

 produced ; but when it came obliquely, the aperture was com- 

 pletely fringed by prismatic tints in their natural order. As 

 it seemed to me that a piece of rubbed glass placed over the 

 diaphragm caused the light to be equally diffused over the 

 interior of the tube, whether it came obliquely or not, I tried 

 the effect of the prism upon it, but the light always seemed to 

 be dispersed in this way as before. (For these experiments I used 

 the apparatus first described, only I took out the object-glass 

 and eye-glasses of the microscope, and looked at the diaphragm 

 through a small prism held in my hands at the ocular end of 

 the body.) In order to have made these latter experiments 

 properly, I should have required a heliostat moved by clock- 

 work, for the motion of the earth subverted the direction of 

 the solar beam. Immediately I had got it truly in the axis of 

 the body of the microscope ; and the power I possessed of 

 counteracting this was only by means of adapting the prism 

 to the motion of the light with my hands.* I should have 

 liked to have dispersed the colour a second time, and fairly 

 produced a spectrum of it on paper under the circumstances 



a great reduction of dispersion, the effect of which is assisted by the fog 

 occasioned by spherical aberration, as well as by the confusion produced 

 by solar light, when I used it as before stated. If there really is achroma- 

 tism, it is confined to a small portion of the centre of the field of view, and 

 even there to a point, which seems to depend much upon the position of 

 the eye; however, I have no doubt that it would be considered such by 

 those whose eyes have not been sharpened by habits of looking into the 

 defects of optical instruments. 



* When the light came quite direct, I of course caught the image of 

 the sun, which dazzled my eyes too much to permit me to be quite cer- 

 tain of the tints or colour I saw. It is very possible that here again there 

 might be only a great reduction of dispersion as in former instances. 



