1287 



line-spectrum AB only a part m the green of tiie length i AB 

 could pass, a granular structure was observed in the immediate 

 neighbourhood of the point P. This structure was quite of the same 

 nature as that which nnder ordinary circumstances was seen near 

 the point 0. At a greater distance from P small sti-ipes were 

 observed along lines directed towards F; the length of these stripes 

 increased with the distance from P. 



The question may be illustrated by a figure. Let CC' be the [)art 

 of the linear spectrum transmittecl by the (liter and let ns consider 



A C C' B P 





the two extreme wavelengths for which the image of the light- 

 point lies at 6' and C' . Let the wavelengths belonging to C and C' 

 be ). and )., so that, A being the red end of AB, 1^ )'. 



Suppose that, working only with the wavelength ;*., we saw a 

 light grain at the point Q. It is eviderit that, working with the 

 wavelength /' alone, We should see the corresponding spot at a point 

 Q' , which is found by drawing C' Q' parallel to CQ and by deter- 

 mining the length of C' Q' by the equation: 



CQ : CQ = I : A'. 



For intermediate wavelengths the light-spots fall between Q and 

 Q' viz., as CC' is relatively short on the straight line QQ' . The 

 production of this line cuts that of AB at the point P, which is 

 determined by the equation 



CP: CP'z=/.:X' (4) 



and which has therefore always the same position, irhichecer strip 

 of light QQ' we may consider. When further, RR.' is a second 

 strip of light, it can be proved, that the lengths QQ' and RR' have 

 the same ratio as the distances Q' P and R' 1\ 



Near P the lines were so short, that they could not be distinguished 

 from "grains". When the wavelength ;. gave a light spot just at /-*, 

 the corresponding light spot of the wavelength X' would coincide 

 with it, as is evident from (4). The existence of a light-spot at P 

 involves of course that without piism we should observe a radially 

 directed line of the length CC' . 



We must observe that the absence of fibres in the immediate 



