Wave-lengths in the Invisible Prismatic Spectrum. 203 



given, most of the observations having been lost through 

 changes of the sky during the course of one determination *. 



Table II. — Experimental Determination of d or n as a 

 function of X (Hilger prism). 



Date of Observation. 



X. 



d. 



n. 



April 1,1882 



A* 



1-010+0-0053 

 1-200+0-0069 

 1-658 +0-0091 

 1-767+0-0094 

 2-090+0-0104 

 2-356+0-0110 



O 1 



46 12 



45 54 

 45 16 



45 08 

 44 45 

 44 25 



1-5654 

 1-5625 

 1-5562 

 1-5549 

 1-5511 

 1-5478 



April 9, „ 



June 27, ,, 



June 13-27, 1882 



July 14, 1882 



June 7, ,, 





We observe that where measures are taken in the prismatic 

 spectrum alone, we can generally use with advantage a bolo- 

 meter of as small an aperture as one fifth of a millimetre ; but 

 that here it is advisable to open it to 2 millimetres, owing to 

 the relative expansion of the spectrum and to the very feeble 

 heat. 



Owing to difficulties arising from the almost infinitesimal 

 amount of heat in question, numerous subsidiary observations 

 are requisite for a single determination, which it therefore 

 takes long to make, each final value resting upon between 20 

 and 100 readings. If it should possibly appear to the reader 

 that, in the three months of consecutive labour which were 

 given to this part of the work, more than six points might 

 have been determined in the curve, he is asked to remember 

 that what is here difficult has till now been impossible. 



Plotting the points given by the data in Table II. and 

 drawing a smooth curve through them, we obtain the curve 

 of "observation" showing n as a function of \ in the lower 

 curve of Plate VIII. 



There would be no gain in accuracy, at this stage, in 

 attempting to work from a formula representing the equation 

 of the curve obtained, as the graphical construction is fully as 

 trustworthy as the data. This I say with special reference to 

 the large original chartsf, which have been drawn by Mr. J. 



* All these observations for discovering the relation between n and A 

 can be conducted with at least as much advantage by a powerful and 

 constant electric light as by sunlight. The latter only, howeyer, was at 

 the obseryer's actual command. 



t These original charts were exhibited to the members of the National 

 Academy of Sciences, at Washington, in April 1883. The engraving here 

 given in illustration, being on a much reduced scale, will merely indicate 

 the exactness of interpolation possible by the originals. 



