Mr. Talbot's Facts relating to Optical Science. 1 13 



filled with this liquid and a candle is viewed through it, in- 

 stead of one, two candles are seen, one of which is red and 

 the other green. It is a very striking experiment, for the rest 

 of the spectrum being wholly absorbed, it in a manner imitates 

 double refraction. Now in ordinary cases, as when this li- 

 quid is examined in a bottle, the two images are superposed, 

 and in the day-light the green tint prevails over the red, 

 while the reverse takes place by candle-light, because the 

 green rays are proportionally more abundant in day-light than 

 in candle-light. 



3. Purple Crystals from a green Liquid. 

 If to a solution of common bichromate of potash be added 

 a mixture of sulphuric acid and alcohol, it is well known to 

 chemists that the liquid changes its colour from yellow to dark 

 green. The compound thus formed is an impure sulphate 

 of chromium. After standing a few hours it deposits an 

 abundant crop of small, roundish, purple crystals. It is im- 

 possible not to recognise in this variety of colour the same 

 peculiar action of chromium upon light which was the sub- 

 ject of the last experiment, but modified by circumstances, 

 inasmuch as here both colours are exhibited by day-light. 



4. A Body in rapid Motion, yet apparently at rest. 



Let a small object be firmly fastened in front of a mirror ; 

 a thread, for instance, tied round the mirror, will do very well 

 for the purpose of exemplification. Let the mirror be made 

 to revolve rapidly, and let the thread be nearly parallel to its 

 axis of revolution, at a little distance from it, however, on one 

 side. Under these circumstances the thread will of course 

 disappear. Now let a candle be placed some feet in front of 

 the mirror, and in the band of light which it causes on the 

 mirror, the thread will be plainly discerned, and apparently 

 at rest. 



This phaenomenon, which I discovered in the year 1826, 

 caused me at the moment, I acknowledge, the greatest surprise. 

 Yet the explanation of it is easy ; for it is only at one fixed 

 period in each revolution that the image of the candle appears 

 to pass behind the thread, and makes it visible. At that pe- 

 riod it can, therefore, be distinguished, but only then, for du- 

 ring all the rest of its revolution it is projected against a dark 

 ground, and invisible from its rapid motion. Is not this ex- 

 periment capable of being applied to some useful purpose ? 

 There are many cases in which it might be desirable to see 

 what passes in the interior or immediate vicinity of a body 

 during the time of its being subjected to a swift motion. 



Third Scries. Vol. 4. No. 20. Feb. 18S4. Q 



