y^n Essay on a hUhcrlo imolserved Properly in Lighf. 183 



surface shall be equally affected, and when they have taken the 

 colour till nothing more is to be gained in point of intensity, they 

 are washed and dried. 



When wool is to be dyed, it should be stamped in the ammo- 

 nlacal bath, and remain there until it has been all equ div im- 

 pregnated ; it may afterwards be wrung very sliglitly and uni- 

 formly; or it may be left to dry of itself. Silk, cotton, hemp and 

 flax require only to be plunged in the dyeing liquor; tliey take 

 the colour very easily. It is jiecessary, however, to wring them. 



The sulphuret of arsenic can give to stuffs all imaginable shades 

 from the finest golden yellow to a marigold yellow. This beau- 

 tiful colour has the precious advantage of lasting for an indefinite 

 time in all its brilliancy; nay, of lasting longer than the cloths 

 themselves. It resists in fact all agents, if we except alkalies; 

 but this inconvenience is well compensated by the other advan- 

 tages which this colour |)iesents. It may be of great advantage 

 in the fabrication of rich tapestries, velvets, and other household 

 stuffs which do not require washing in soap, and in which fixity 

 of colour is one of the most precious qualities. 



I think that the moderateness of the price of this dye, and the 

 extreme simplicity of its application, will induce dyers to make 

 use of it, and that it will tliereby become an interesting acquisi- 

 tion to the art of dyeing. The ammoniacal solution of the sul- 

 phuret of arsenic may also be employed in the fabrication of co- 

 loured papers. 



Nancy, Dec. 31, 1819. 



XXVII. jin Essay on a Property in Light tvhich hitherto has 

 ■ been unobserved by Philosophers . By Captain Forman, 

 R.N. 



It is a principle in catoptrics, that when light is reflected from 

 any body, the angle of reflection is always equal to the angle of 

 incidence; and this principle is proved to be true, because it al- 

 ways corresponds with facts. A person, for instance, standing 

 before a looking-glass, in the line of a reflecring angle, sees the 

 image of the flame of a candle (or any other object when there 

 is light) expressed upon its surface ; but he can only see it wiien 

 he is looking in the direction of that line, because the rays are re- 

 flected from every other part of the glass in angles that do not 

 come near his eye. 



Now, if there was no other light derived from a luminous 



body, bciides these primary rays (which is all that philosophers 



have supposed), the greatest part of objects would be invisible, or 



M I at 



