OF ARTS AND SCIENCES. 419 



micrometer, and intei-polate so as to obtain the index of a coincident 

 line by measuring the distance of the rekitive hne D from one or more 

 visible lines in the comparison-spectrum. The eye-piece micrometer 

 suggested by Professor Rood * would also give all necessary precision, 

 and would have the advantage of being very much cheaper than a 

 filar micrometer. 



The method above aiven enables us to determine the index of re- 

 fraction of a single line only, unless the prism is emptied, cleaned, 

 dried, and the operation then repeated with a second selected line. To 

 obviate this ditficulty, I have employed the following modification of 

 the prism with entire success. The prism is divided into two by a 

 septum perpendicular to its refracting edge. Each prism thus formed 

 has an opening in its base by which liquid may be introduced or 

 removed, and which can be closed with a cork. "When the two glass 

 plates are carefully cemented to the brass frame, the two prisms will 

 have the same refracting angle. One of them is then to be tilled with 

 the comparison-liquid, the other with the liquid the indices of which 

 are to be determined. The double prism being now placed upon the 

 stage of the spectroscope, one face of the prism containing the com- 

 parison-liquid is to be covered with a slip of metal. The spectrum of 

 the li(|uid to be examined will then be seen by means of the observing 

 telescope. Any line — as, for instance, G — may then be selected, 

 brought into the position of minimum deviation, and the telescope 

 adjusted until this line bisects the space between the parallel wires in 

 the plane of the diaphragm of the eye-piece. The telescope is then to 

 be clamped as before without disturbing the adjustment. If now one 

 face of the prism containing the liquid examined be covered with the 

 slip of metal removed from the face of the other prism, the spectrum of 

 the comparison-liquid will be seen, and it will be easy to determine what 

 line in this spectrum most nearly corresponds in ^jositioa to the line G 

 of the other spectrum. By alternately covering the faces of the two 

 prisms with the metal slip, coincidences or near coincidences may be 

 observed for D, E, F, &c. ; and in this manner the data obtained for 

 the constants in Cauchy's dispei'sion-formula for the liquid examined, 

 in a short time and with great facility. It must be borne in mind 

 that the two specti"a in this process cannot be seen simultaneously, 

 their images being combined by the observing telescope into one.f 



* Am. Journal, 3d series, vol. vi. p. 44. 



t Mr. S. P. Sharpies has suggested to me that if a cylindrical lens were em- 

 ployed as the object-glass of the observing telescope, the two spectra could be 

 seen in the field at the same time. 



