290 Cooper on the Fixed Lines 



lation of Fraunhofer's lines; and as I have never seen, in any 

 work, the precise method described that is necessary to be 

 employed for their successful production, it occurred to me 

 that a description of the instrumental means I adopt might 

 not be unacceptable to some of your readers, and enable those 

 of them who have more opportunity than myself, and are desi- 

 rous of pursuing the investigation into the cause of their pro- 

 duction, to proceed in their researches without the loss of time 

 consequent on the experiments of any kind, where everything 

 has to be sought. It is, therefore, with this view that 1 have 

 written, and endeavoured, if possible, to save a portion both 

 of time and expense to those who may feel inclined to enter 

 this rich field of inquiry, and to enable those who may wish 

 to determine the refractive and dispersive powers of substances 

 to do so with precision ; and, should you concur with me in 

 the propriety of these suggestions, to request you to give them 

 insertion in your useful publication. 



In article 422 of Mr. Herschel's admirable paper on Light 

 in the Encyclopaedia Metropolitan a, he says that ' with glass 

 prisms of our manufacture it would be quite useless to attempt 

 the experiment.' An assertion coming from such high autho- 

 rity is of itself sufficient to deter any one from making the 

 trial ; and he recommends the substitution of hollow prisms, 

 filled with highly refractive media, in lieu of the glass prism. 

 With this assertion I am in some degree disposed to coincide, 

 but certainly not to the extent he there intimates. True it is 

 that not one prism in twenty is fit to be employed for the pur- 

 pose; yet I have obtained several of British glass, both of flint, 

 plate, and crown, and of various refracting angles, that have 

 shown not only the most prominent of the lines, but even those 

 that may be considered as of the second and third order, and in 

 such abundance, under favourable circumstances, that it would 

 be no easy matter to count them : suffice it to say, that a good 

 prism is necessary, and such can be met with, though not with- 

 out some difficulty ; but neither its size, as respects its length, 

 the breadth of its sides, the refracting angle, nor the kind of 

 glass of which it is made is a matter of much moment*; yet 



* I have in my possession equilateral prisms of flint, plate, and crown glass, 

 which are only three-quarters of an inch long, and the sides less than three-tenths 



