1865.] Mr. Gassiot Description of a Rigid Spectroscope. 321 



from changes in the temperature were ascertained to be so variable that no 

 reliable result could have been obtained. 



These preliminary observations were nevertheless so far valuable, for 

 they proved that changes of temperature were taken up very slowly by the 

 prisms, and that it would be consequently useless to employ the instrument 

 in balloon ascents where rapid fluctuations of temperature would continu- 

 ally occur. 



I then determined to attempt the construction of a rigid spectroscope 

 with which observations might be made either on board a vessel or on 

 land, in various latitudes ; and as the question of the total weight of the 

 apparatus became no longer of paramount importance, Mr. Browning de- 

 cided on mounting the instrument in cast iron. The adjustments of the 

 telescope being dispensed with, it was mounted in two cast-iron blocks, and 

 fixed on a bed of slate; the prisms, with their adjustments, were attached 

 to an iron plate, the plate being bolted to the same slate-bed. In this 

 arrangement the observations still showed discrepancies, which were con- 

 sidered to arise from changes in the adjustments of the prisms, produced 

 by alterations of temperature. Mr. Browning then removed all the ad- 

 justments of the prisms, and also the iron bed-plate, bolting the prisms on 

 the bed of slate, and securing their correct position by filing and scraping. 

 Full particulars of this arrangement will be found in the description of the 

 apparatus. 



The instrument has been carefully examined by Mr. Stewart, not only 

 at Kew Observatory but also from time to time during the progress of its 

 construction, as well as after it was completed in Mr. Browning's work- 

 shops ; and it may now be considered that, with ordinary care during its 

 transit from place to place, any observations made with it can be depended 

 on as far as the mechanical arrangement is concerned. 



I am indebted to Mr. Browning for the description of the apparatus, 

 with the notes of the readings as they were made by himself. 



The optical arrangement is as follows : In order to obtain great re- 

 fractive power in a moderate compass, the prisms were arranged as in 

 Plate VI. fig. 1. P and P' represent two prisms of heavy flint glass, having 

 sides 2| inches high, and 3 inches long. These prisms have refracting 

 angles of 45. They are arranged at the minimum angle of deviation for 

 Fraunhofer's line D. E. represents a prism of similar material and dimen- 

 sions, but with a refracting angle of 22 30', that is half P and P'. 



The dense flint glass of which these prisms are composed was made by 

 Messrs. Chance Brothers and Co. It has a specific gravity of 3 '9. Its 

 mean refractive index is 1*665, and its dispersive power 0-0752. The 

 prism R has the side further from P and P' silvered. The nearest side is 

 placed at the same minimum angle of deviation from P as P is from P'. 



D andD' represent a compound prism, formed by cementing a very small 

 diagonal prism D' on to a large diagonal prism D with a transparent 

 cement, in such a manner that two of the plane surfaces are parallel. They 



