Mr. Talbot's Pacts relating to Optical Scie?uc 40.'5 



black spot, as small and well defined in appearance as a full 

 point in a printed book, being a curious instance of the well- 

 known fact, of the interference of rays of light producing 

 darkness. This black spot was seen in all the circles at once, 

 in consequence of their having equal diameters. 



4. When the copper-plate was placed obliquely and held 

 in various positions, a great variety of very singular patterns 

 were displa3'ed, which can be compared to nothing so well as 

 to tissues woven with threads of various colours. It would be 

 impossible to describe these, any more than the ever-changing 

 figures of the kaleidoscope. They seem to vary ad iiifiiiitum, 

 and in whatever position the plate is placed, they appear al- 

 ways as distinct as if they were in the focus of the lens. 



5. In most optical experiments it is essential that vision 

 should be performed along the axis of the lenses which are 

 employed, or very nearly so. But in these experiments this 

 singularity occurs, that the lens may be placed in any posi- 

 tion ; so that when held even very obliquely the only effect is a 

 considerable alteration in the pattern, which in other respects 

 remains as distinct to the eye as before. The experiments 

 hitherto related, are some which 1 had the pleasure of show- 

 ing to some distinguished members of the British Association 

 a siiort time previously to the late meeting at Bristol ; and are 

 communicated in the hope that they may prove interesting to 

 the cultivators of optical science. 



§ 3. Remarkable Property of the Iodide of Lead. 



This substance possesses a property of a singular nature, 

 which I believe differs from anything previously described ; 

 or if it is reducible to known laws of chemical and molecular 

 action, offers at least a very striking and beautiful example of 

 them. 



If a solution of acetate of lead is mixed with a saturated 

 solution of hydriodate of potash, and the'mixture well stirred, 

 the iodide of lead which is formed in abundance, though at 

 first yellow, speedily grows pale, and afterwards becomes 

 perfectly white. If a small quantity of this is taken when 

 iVeshly made and moist, and squeezed between two plates of 

 glass, it may be seen by the help of a microscope to be en- 

 tirely composed of very delicate capillary crystals; and if in 

 this state it be laid aside, I do not find that it undergoes any 

 change after being kept several nionths. 



But if, while fresh, il be warmed over a spirit-lamp, it sud- 

 denly turns yellow, the first impression of the heat being suf- 

 ficient to proilute that cfictt. As soon as this happens, it 

 should be removed from the lamp and again examined with 



