OPTICS. 



29 



of producing hmf. mid which had a less 

 I'isibility titan red light. This re- 

 sult was confirmed by th its of 

 Sir II. Englefield, a-ll also by Sir Hir.n- 

 pliry Daw, wiu> repea'e.l 'the experi- 

 ments in the rays of an Italian sun, and 

 by nivMiis of tlK-nn.MiiJiers with minute 

 bulbs. Sir H. Engleiield obtained the 

 following results : 



Blue. 



Green 



Yellow . . . . 







Beyond Red 



Temperature. 



. 56' 

 . 58 



62 



72 



79 



The prisms, by which all these expe- 

 riments were made, were, we believe, of 

 flint orlass. It has been recently pr 

 by M.Seebeck*, that the heating power 

 of the colours of the spectrum depend 

 upon the substance of which the prism 

 is made. Thus : 



In Waterrthe greatest 



lieat is in the Yellow. 



Sulphuric AcidJ Orange. 



Crown or Plate Glass. . . . Middle of Red. 

 Flint Glass Beyond the Red. 



Deoxidating power of the spectrum. 

 In the year isoi. the' late Mr. Ritter, 

 of Jena, discovered that the rays of the 

 spectrum had different chemical proper- 

 ties which resided in the violet end of 

 the spectrum, and existed even beyond 

 the violet light. Muriate of silver, for 

 example, became black beyond the vio- 

 let rays ; a little less black in the violet ; 

 and still less in the blue. Dr.Wollaslon 

 made the same discovery about the same 

 time. In repeating these experiments, 

 Dr. Seebeck found, that the- colour of 

 muriate of silver varied with the coloured 

 spaces in which it was placed. In and 

 beyond the violet, it was reddish brown ; 

 in the blue, it was blue or bluish u'lvv : 

 in the yellow, it was unchanged white, 



or faintly tinged with yellow ; and in the 

 re.l, it was red. With prisms of flint 

 - the muriate of silver also became 

 red at a spot entirely beyond the re J. 



M i^>i ^fi*irt poto?r of the s-ifar rays. 

 Nearly JU years ago, Dr. Morichini, of 

 Rome, found that the violet rays of the 

 spectrum had the property of communi- 

 cating magnetism ; but this result was 

 involved in doubt, and by many philo- 

 sophers entirely discredited, till it was 

 established by some recent experiments 

 by Mrs. Somerville. Having covered 

 half of a sewing needle, about an inch 

 long, with paper, she exposed the other 

 half for two hours to the violet rays. 

 The needle had then acquired north po- 

 larity. The indigo rays produced n, 

 the same effect ; and the blue and green 

 ray* produced it in a still less degree. 

 In the yellow, orange, red, and invisible 

 rayv. no magnetic influence was exhi- 

 bited, even though the experiment was 

 continued for three successive days. 

 The same effects were produced by in- 

 closing the needle in blue or green 

 glass, or wrapping it in blue and green 

 ribbands, one half of the needle being 

 always covered with paper. 



Illuminating power of the spectrum. 

 Dr. Hersehel has represented, by a 

 curve, the gradual shading off of the light 

 from the yellow space to the red and 

 violet extremities of the spectrum. A 

 series of experiments on this subject 

 were made with great care by the laie 

 M. Fraunhofer of Munich/ and the 

 results of them are exhibited inyfjg-. 33. 



In this figure A I Q P represents the 

 prismatic spectrum, and the lines A 1, 

 13 2, Sec., bounded by the curve line 1, 

 2, 3, 4, 5, &c., represent the degree of 

 light at the points A, B, C, Dr&c. of 

 the spectrum. In numbers they ;v 

 follow, the intensity of the light of the 

 brightest point being 1 . 



Fig. 33. 



* See t>e Edinburgh Jo.rrnal of Science, No. i. p. 353. 



\ M. Wnnch found that in alrolwl au.loil'of turpentine the greatest heat was in'the yellow 



J And also in solutions of sal-ammoniac and corrosive sublimate. 



