410 J. RHEINBERG ON POLARISED LIGHT EFFECTS. 



forming the admixture, was employed many years ago by Helm- 

 holtz, who gives a description of an elaborate slot mechanism 

 suitable to this purpose in his " Handbuch der Physiologischen 

 Optik," p. 353. Reference to slots used in a similar manner is 

 also made in " Modern Chromatics " by Ogden N. Rood. Although 

 so far, therefore, there is nothing new about the slot arrangement, 

 it would appear that the method has hitherto only been used with 

 prisms, I cannot find any record of gratings having been employed. 

 The distinctive feature of the latter is that the spectra with the 

 orders of then* colours reversed can be superposed, as shown in 

 the diagram, whilst with a prism the superposed spectra would 

 both have their colours in the same order, i.e. their red ends both 

 on the same side. The former arrangement has an obvious 

 interest of its own because of the comparison it afibrds of colour 

 sensation produced by a series of pairs of colours, each pair of 

 which has the same mean wave-length. 



Jonni. Quekett Microscopical Club, Ser. 2, Vol. VII., No. 47, November 1900. 



