*!jZmIi, Feb.Tim'] li^oijal Microscopical Sociehj. 71 



numbers representing them will enable observers at a distance from 

 each other to compare their results, or even to work simultaneously 

 on the same subject. The apparatus is here in operation this 

 evening. 



Dr, Lawson has suggested that it is a great advantage of this 

 contrivance that it does not monopolize one of the two spectra, ^s 

 is the case with the use of the quartz scale : for in describing two 

 spectra, only slightly differing from each other, it may be used at 

 once to determine the difference between them. Many substances 

 give two different spectra when examined by transmitted or reflected 

 Hght, though there is a generally close resemblance between them. 

 This was the case in the instance of the dichroic fluid, kindly given 

 me some time since for examination by our President, the Kev. J. B. 

 Reade. 



At the time I described the two spectra of this strange fluid 

 before the Society, I felt a want of some simple method of indi- 

 cating the differences between them. I hope to exhibit very soon 

 before the Society coloured forms of blank spectra, that is, free from 

 lines, as I think publishing these at a low price will facihtate the 

 adoption of the plan and the use of the instrument. 



