OF ARTS AND SCIENCES. 417 



II. — ON A METHOD OF MEASURING REFRACTIVE INDICES 

 AVITHOUT THE USE OF DIVIDED INSTRUMENTS. 



The importance of an accurate determination of all the physical con- 

 stants which characterize any substance having a definite chemical 

 constitution becomes daily more and more evident. The researches of 

 Gladstone, Landolt, and others have shown that indices of refraction 

 possess a peculiar value and interest. As the instruments necessary 

 for their determination are expensive, and often beyond the reach of 

 working chemists, a simple and sufficiently accurate method of meas- 

 uring them by means of the spectroscope alone will doubtless be wel- 

 comed. 



The method which I propose is one of comparison, and applies with 

 convenience only to the case of liquids. A hollow prism is to be filled 

 with the liquid to be examined, placed upon the stage of the spectro- 

 scope, and turned until a given ray — the line D, for instance — is seen 

 by the observing telescope to be in the position of minimum deviation. 

 The eye-piece of the telescope should have two parallel spider-lines 

 placed very near each other in the plane of the diaphragm. When the 

 dispersion is sufficiently great to separate the line D into its two com- 

 ponents, either component may be made to bisect the interval between 

 the two spider-lines, or the two components may be made to occupy 

 such positions that their middle line shall bisect the interval. The 

 observing telescope is then to be firmly clamped. The prism is now 

 to be removed, the liquid poured out, and the prism cleaned and dried 

 carefully. It is then to be filled with any liquid the indices of refrac- 

 tion of which are known, and which the observer judges to have a 

 mean index not greatly differing from that of the liquid to be meas- 

 ured. The prism is to placed upon the stage of the spectroscope, and 

 turned until the observer ascertains that the two spectra would be in 

 the field of view if both could be seen at the same time ; or, what is 

 the same thing, that they would be more or less completely super- 

 posed. Should this not be the case, another comparison-liquid must be 

 chosen ; and so on until one is found which fulfils the requisite condi- 

 tions. Supposing that this is successfully accomplished, the prism is to 

 be turned until, for the position of minimum deviation, a known line 

 in the spectrum exactly bisects the interval between the two spider- 

 lines. The index of refraction of the given liquid for the line D is 

 VOL. X. (x. s. II.) 27 



