2ff Mr. Walgon*s Mode of exhibiting the Colours of thin Plates. 



I find the addition of a little hydrochloric acid to the solu- 

 tion of the bichloride of mercury renders the precipitated 

 triliodide of mercury more permanent in the air. 



Robert Hunt. 



" ■ ' ' ' ■ ~~ " " " ' =^-^ 



VII. A simple Mode of exhibiting the Colours of thin Plates. 

 By Mr, James Walgon. 



To the Editors of the Philosophical Magazine and Journal, 



Gentlemen, 



ALTHOUGH Newton's rings have often been described, 

 I believe they have been but rarely seen. The reason 

 is, because few persons have lenses, clamps, and screws suit- 

 able for Newton's experiment. In your Journal for the 

 month of March in the present year, (L. & E. Phil. Mag., 

 vol. X. p. 183,) there is an article by the late Dr. Ritchie, en- 

 titled, " A simple mode of exhibiting Newton's rings." Two 

 circular pieces of thin plate glass, separated at the circumfe- 

 rence by a single gold-leaf, are to be used instead of lenses ; 

 but in order to bring the glass plates to touch in the centre, 

 we must have " a rectangular frame of iron or brass, and a 

 screw." I am afraid, therefore, that Newton's rings, as an ex- 

 periment for illustrating the colours of thin plates, are not likely 

 to be often visible, except in diagrams. 



I shall now proceed to describe a very simple mode of ex- 

 hibiting the colours of thin plates by means of an experiment 

 which requires no expensive apparatus. 



We have all seen the brilliant colours which are reflected 

 from the narrow cracks in mica, and there are little concen- 

 tric coloured rings in this mineral, which may be found by 

 using a magnifier. It occurred to me lately that possibly 

 these splendid colours might be made to appear in the mineral 

 whenever required. To effect this I obtained a thin plate or 

 film of air, by introducing a lancet into the edge of a clear 

 plate of mica, carefully separating the laminae to the extent of 

 about one inch square. Then holding the plate of mica with 

 both hands, and pressing the middle finger of the right hand 

 just under the spot where the film of air was made, I was much 

 gratified by the appearance of several beautiful curved lines 

 or bands of different colours, which followed the direction in 

 which I moved my finger. These curved lines or bands in 

 their forms very much resemble those of a fortification agate. 

 The perfect rings are small, but they expand or contract ac- 

 cording to the degree of pressure. The colours also change 

 with the pressure of the finger, and when the finger is al- 



