82 Prof. Powell on the Theory of the dark Bands 



(4.) My attention was drawn to the subject in the course 

 of last summer, when I repeated the experiments, and devised 

 several new modifications with reference to an explanation 

 which it appeared to me was supplied by the undulatory 

 theory ; to these investigations I referred briefly at the Bir- 

 mingham meeting of the British Association, 1839. For se- 

 veral reasons (on which I need not here enter) I have delayed 

 publishing any details ; nor should I do so now, but that ha- 

 ving learned that Mr. Airy has recently pursued the research 

 to many entirely new conclusions*, I am anxious to put on 

 record the few points I have been able to establish, and to 

 vindicate my views from misconceptions to which they have 

 been exposed. 



(5.) The following distinctions are important to be borne in 

 mind with reference to the explanation of the phenomena. 



In these experiments we have to consider the different ele- 

 mentary pencils of which the spectrum, as presented to the 

 eye, is formed ; and with respect to each of these, in the case 

 of the prismatic spectrum it is easily seen that the edge of the 

 plate intercepts that half which lies towards the EDGE of the 

 prism. 



In the interference-spectrum (according to Fraunhofer's 

 method), the spectra are formed one on each side of the axis, 

 with their violet ends towards it. The edge of the plate in 

 this case must always intercept that half of each primary 

 pencil, which after passing the focus lies NEAREST to the axis. 



(6.) I have found that with the same prism the intercept- 

 ing plate must be within limits of thickness, which differ ac- 

 cording to the substance of the plate, and with the same plate 

 the character of the bands differs with the medium of which 

 the prism is formed. These differences appear to depend on 

 the refractive and dispersive powers of the substances. 



(7.) With a prism of flint glass and a plate of mica, the 

 greatest thickness which can be used may be about the ytnjth 

 of an inch. In this case the bands appear fine and numerous, 

 and it seems only in consequence of their increase in number 

 that they cease to be distinguishable when the thickness is 

 increased beyond this. 



If we use less thicknesses (such as those into which mica 

 is easily split) the bands become broader and fewer, and at 

 length faint and ill-defined. It is perhaps not possible to 

 distinguish them if fewer than four or five are formed through- 

 out the spectrum. The bands are never very dark ; show- 

 ing that only a portion of the rays is concerned in their for- 

 mation. 



[* A notice of Mr. Airy's paper on this subject will be found in our re- 

 port of the proceedings of the Royal Society for June IS, 1840. EDIT.] 



