110 Mr. J. P. Cooke on a Spectroscope. 



interposed air and renders the vapour of the water it contains 

 negative by friction. The path is thus open to the electric cur- 

 rent, the access of which to the negative knob is facilitated by 

 the expansion of the layer of surrounding air. Hence Ruhm- 

 korfFs machine differs from the electric battery, in which the 

 striking-distance does not vary, either according to the circum- 

 stance that one of the armatures is positive or negative, or ac- 

 cording to the nature of the connecting arc*. 



On using as the discharger two wires of the same size and 

 shape, M. de Moncel had remarked a preponderance of the 

 positive rheophorus of the induced current, by the interposi- 

 tion upon this rheophorus of a very powerful resistance, as that 

 of the secondary circuit of a Ruhmkorff 's machine f. The various 

 experiments of which I have given a summary in this memoir 

 prove that the form of the ends of the discharger may be very dif- 

 ferent, and that the addition of a resistance towards one of the 

 ends of the induced wire is not requisite to produce a dispa- 

 rity in the manner in which the discharge traverses a constant 

 interval in the free air. 



XV. On the Construction of a Spectroscope with a number of 

 Prisms, by which the angle of minimum deviation for any ray 

 may be accurately measured and its position in the solar spec- 

 trum determined. By Josiaii P. Cooke, Jun.% 



IN an extract from a letter of the author published in Silliman's 

 Journal, vol. xxxvi. p. 266, a method of adjusting the prisms 

 of a compound spectroscope was described, by which the adjust- 

 ment for any portion of the spectrum could be obtained with 

 great rapidity and accuracy. A further study of the subject 

 has shown that the method of adjustment then only briefly 

 described admits of the highest precision, and may be applied to 

 the exact measurement of the angle of minimum deviation of 

 the spectrum-rays. It has been thus possible to apply the great 

 dispersive power of the large Cambridge spectroscope in deter- 

 mining the relative position of the various spectrum-lines, and 

 to secure all the accuracy of which angular measurements are 

 capable. The value of such measurements is obvious, and, with 

 the hope that this method will prove to be an assistance to inves- 

 tigators, we propose to give in this paper a description of our 

 instrument and of the manner of using it. 



* Riess, op. cit. vol. ii. pp. 80 & 130. 



t Notice sur Vappareil de Ruhmkorff, 4th edit. p. 248 (1859). Re- 

 cherches sur la non-homogeneite de IVtinceUe d" induction, p. 89 (1860). 

 t From Silliman's American Journal for November 18()5. 



