44 On Madder, and 



If a disk, composed of two semi-circles, one white and 

 the other black, be viewed, while in motion, from behind 

 the revolving slitted disk, the diameter of the disk will 

 vibrate on both sides, the centre being fixed ; the white 

 gaining upon the black and the black, upon the white, and 

 so on, alternately. 



The cause, then, of the appearances detailed in the first 

 part of this paper, is the same as in Mr. Wheatstone's 

 experiment, the light comes and goes before the disk has 

 time to move through any sensible space ; but, in the 

 experiments where the light of a lamp flashes upon the 

 painted disk through the slitted disk, or where the eye is 

 placed behind the slitted disk, the duration of the light is 

 greater than the electric light, or than that from phosphu- 

 retted hydrogen, &c, and the disk does pass through a sensible 

 space. Now, as the circumference of the disk moves quicker 

 than the centre, that is, the velocity decreases from the 

 circumference to the centre, a black space, for example, 

 seen at one point of the circumference, will have moved 

 through several degrees as the slit passes the eye ; while, 

 at or near tbe centre, the space gone through is barely 

 appreciable. This, together with the persistance of impres- 

 sions on the retina, added to that which is said above, will, I 

 think, account for the revival of the radii, as also for their 

 curvature; and the rapid succession of black and red spaces 

 will account for the apparent increase in their number. 



If the distance between the two disks be considerable, 

 fourteen or fifteen feet, for instance, the curvature of the 

 radii will be corrected, and their number will not be aug- 

 mented ; because, a full view of the disk is thus obtained, 

 and the relative velocities of the centre and circumference 

 compensated by an impression of the whole of the disk 

 being formed upon the retina. 



Salisbury, ]$th November, 1835. 



Article V. 



On Madder, and Madder Dyeing. 



(Continued, from vol. ii. page 457.) 



Characters of Madder-Purple. — When cautiously heated in 

 a glass tube, madder-purple melts into a dark -brown viscid 



