6o 



NA TURE 



[Nov. 2o, 1884 



more apparent as we proceed in our examination of M. 

 Rosenstiehl's work. 



For the investigation of colours, or rather of colouring- 

 matters, the author employs concentric disks, which are 

 kept in rapid rotation by mechanical means of the simplest 

 character. These disks may be coloured uniformly or in 

 sectors of various hues ; the well-known result of the rapid 

 rotation is in the latter case the mixture of the sensations 



velvet. The admixture of black is then produced by 

 cutting from the disk a sector ; the disk being then placed 

 before this chamber adds the sensation of black to that of 

 colour in the proportion of the size of the sector removed 

 to that of the disk. A special apparatus is described for 

 measuring the sectors of the disks, as is also the very 

 necessary instrument by which the disks are cut with or 

 without the simultaneous removal of sectors. 



Fig. 4. 



of light, then fusion into a single uniform coloured im- 

 pression. In the study of the degradation of colours by 

 admixture, it is necessary to have both a black and a 

 white. The white is obtained by means of precipitated 

 barium sulphate applied to a suitable surface ; the black 

 is obtained by means of a small chamber before which 

 the disks rotate, this chamber being lined with black 



The author's experimental results are given in a series 

 of coloured plates, to which descriptive notes and expla- 

 nations are attached. Some of these we proceed to repro- 

 duce. Plate 1 in the book is a study of complementary 

 colours or hues. The superposed disks are represented 

 both at rest (Fig. 1) and in motion (Fig. 2) ; the mechanical 

 details, such as the attachment of the disks to the re- 



