214 



REPORT — 1877. 



placement in those fractions ; but these must be reserved for the second part 

 of the report. 



The examination of the optical properties of the several fractions presents 

 some details of considerable interest. When the crystals are examined 

 through the two opposite axes, a change of colour in the several fractions 

 may be observed to take place in the following order down the series : — 



It may be seen from this Table that the colours shown through the dif- 

 ferent axes pass in a direct order down the spectrum in each column. 



In the first fractions the more highly refractive rays of the cobalt spec- 

 trum mingle with the green of the nickel, whilst in the last the two rays are 

 those adjacent to each other in the cobalt spectrum only. 



It is worthy of remark also, that as the cobalt spectrum consists of all the 

 colours except the green, the nickel spectrum consists of none but the green 

 rays. 



Differences in the ratio of nickel and cobalt, which would be detected only 

 by very carefully conducted analyses, can be remarked by the distinct and 

 differing dichroism when the crystals are examined by the dichroiscope. 



That these fractions are not mere isomorphous mixtures, as is generally 

 understood by the word, we think is shown from the fact that large crystals 

 taken for analysis exhibited the same dichroism throughout. It thus becomes 

 interesting to determine, if possible, how far the phenomenon of dichroism 

 is dependent on, or consequent upon, molecular constitution ; in this direc- 

 tion some preliminary experiments have been made to see whether crystals 

 could be stained in such a manner as to show dichroism by the influence of 

 their impurities. It is not easy to secure the necessary conditions without 

 the possibility of forming molecular compounds ; but results have been ob- 

 tained which when thoroughly examined will be reported. 



Wo may mention that from the ease with which the fractions maybe pre- 

 pared, and from the great variety in their dichroism, they form an excellent 

 series for the study of that phenomenon. 



