418 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



To which qualities were added, when assisted however by certain more 

 intense illuminations, a transparency nearly as great, and a definition in certain 

 parts of the spectrum considerably better, than I have seen in almost any 

 smaller instrument. 



For this latter excellence I have chiefly to thank Mr Adam Hilger, and to 

 laud the extreme skill and perfection wherewith he constructed both the glass 

 cores of the bisulphide prisms, and their all-important anti-prisms out of 

 admirably hard, white, and uniform crown glass. The Micrometrical recording- 

 apparatus was exquisitely constructed by Messrs T. Cooke & Sons of York, 

 to whom I am therefore much obliged; while I am further quite unspeakably 

 indebted to the progress of Chemical Science, which has, in recent years 

 entirely freed bisulphide of carbon from the horrors of its ancient smell, and 

 has given us a fluid perfectly colourless, nearly inodorous, exceedingly trans- 

 parent, and endued with less refraction but more concomitant Dispersion, than 

 anything else under the Sun. 



In discussing therefore the results obtained with all these advantages, I 

 shall not only refer to MM. Angstrom and Thalen's now nine year old, yet 

 still most excellent, essay, but shall consider it a duty to seek out later and 

 more advanced works, if they exist, elsewhere. Paying particular attention to 

 the recent Reports of the British Association's very powerfully constituted 

 Committee # for reporting on our present knowledge of Spectrum Analysis in 

 1880, 1881, 1882, and 1883. 



For their admirable digests of all that has gone before, and all that has 

 come up to their own time, in the now voluminous bibliography of the 

 spectrum, enables every one to locate the place, and assign the value of any 

 new observation both with high certainty and the least loss of time. 



PART I. 



The First Gas to be Examined, and under what Conditions. 



As my object has been to a large extent to pass over superficial variations, 

 and arrive, if possible, at the great constants of Nature in this department,— 



* The Members of the Committee are given thus- 



Professor Dewar. 

 Dl Williamson. 

 ])r Marshall Watts. 

 Capt. W. de W. Abney, RE. 

 Mr StONBT. 

 Professor HARTLEY. 

 Professor M'Leod. 

 Professor Carey Foster. 



Professor A. K. Huntington. 



Professor Emehson Reynolds. 



Professor Reinold. 



Professor Liveing. 



Lord Rayleigii. 



I)r Schuster, and 



Mr W. Chandler Roberts, Secretary. 



