68 Transactions of the ["t^^.T, V^n^^"' 



I Journal, Feb. 1, 1870. 



II. — On a Method of Measuring the Position of Absorption 

 Bands ivith a Micro-spectroscope. 



By John Browning, F.E.A.S., F.R.M.S. 



{Read before the Royal Microscopical Society, January 12, 1870.) 



As the micro-spectroscope is constantly receiving new applications, 

 both practical and scientific, thanks to the untiring ingenuity of 

 Mr. Sorby, it is daily becoming more desirable that we should have 

 a simple and accurate method of measuring the position of lines, or 

 bands, in absorption spectra. It is true that Mr. Sorby has con- 

 trived an arrangement founded on strictly scientific principles, a 

 method which overcomes the great optical difficulty arising from 

 the irrationality of dispersion, and which enables observations and 

 measm-ements made with different instruments and by different 

 observers, to be compared successfully with each other. 



But Mr. Sorby's ingenious and accurate method of measurement 

 does not find that favour with manipulators to which it appears to be 

 entitled. There are various causes for this. It wiU be recollected that 

 Mr. Sorby's plan consists in employing a thin plate of quartz placed 

 between two Nicol's prisms. On bringing this arrangement in 

 front of the slit of the micro-spectroscope, bands are produced in 

 the spectrum by the interference of light, and the apparatus is con- 

 trived to give a number of bands that will divide the visible spectrum 

 into twelve parts. The principal objections to this beautiful con- 

 trivance are, that it is an artificial scale, that it is very difficult of 

 construction, and that if the quartz plate gets injured it cannot be 

 repaired or replaced by any but a specially-trained workman, who 

 understands the whole arrangement, and how it has to be used with 

 the instrument. 



Beyond this, as no figures, pointers, or other distinguishing marks, 

 can be made to appear in the field of view by the side of the spec- 

 trum, frequent mistakes are made in counting the lines, particularly 

 those near the middle of the spectrum. Urged repeatedly by many 

 persons, and especially by Mr. Hogg, to undertake the task, I have 

 contrived an arrangement which seems to avoid all the objections 

 just enumerated, though it may be open to others which I have not 

 foreseen. 



I will now describe the apparatus, and then conclude with a brief 

 account of the simple plan which I propose should be adopted to 

 make all measurements taken with instruments of this kind com- 

 parable with each other. 



Fig. 1 represents the upper part of the micro-spectroscope. 

 Attached to the side is a small tube. A, A. At the outer part of 

 this tube is a glass plate, blackened with a fine clear white line 



