OPTICS. 



secondary spectrum, arise from the irra- 

 tionality of the coloured spaces which 

 we have explained in the preceding 

 chapter. The achromatic telescope, 

 therefore, required still further improve- 

 ment, in order to get rid of these se- 

 condary colours ; and science is indebted 

 to Dr. Blair of Edinburgh, for a most 

 beautiful method of accomplishing this 

 great object. Having observed that 

 when the extreme red and violet rays 

 were perfectly united, the green were 

 left out, or were the outstanding rays, 

 as shown in /?#. 31, where the red and 

 violet rays F V, F R are perfectly united 

 at/, while the green rays are more re- 

 fracted, and cross the axis at g t he con- 

 ceived the idea of making an achromatic 

 concave lens, which should refract the 

 green less than the united red and violet, 

 and an achromatic convex lens which 

 should do the same; and as the concave 

 lens refracted the outstanding green to 

 the axis, while the concave one refracted 

 them from the axis, it foDowed, that by 

 a combination of these two opposite 

 effects, the green would be united with 

 the red and Violet. 



By means of two fluid media and three 

 glass lenses, Dr. Blah* constructed a tele- 

 scope in which he could not discover any 

 colour by the most rigid test. 



As this, however, was a complex 

 combination of lenses, Dr. Blair still 

 sought for some single fluid which 

 should produce the effect when com- 

 bined with glass, and he fortunately dis- 

 covered that the muriatic acid mixed 

 with a metallic solution answered his 

 purpose. In the spectrum formed by 

 this fluid, the green were among the 

 most refrangible rays as shown in fig. 

 30 ; and when its dispersion was cor- 

 rected by that of glass, there was pro- 

 duced an inverted secondary spectrum, 

 that is, one in which the green was 

 above, when it would have been below 

 with a common medium. He, there- 

 fore, placed a concave lens of muriatic 

 acid with a metallic solution between 



Fig. 32. 



two lenses of glass, as shown inyfc. 32, 

 and he informs us that with this object- 

 glass the rays of different colours were 

 bent, from their rectilineal course with 

 the same equality and regularity as in 

 reflexion. Through telescopes of this 

 description, only 12 and 15 inches lonfr, 

 Professors Robison and Playfair saw 

 double stars with a degree of perfection 

 which astonished them*. 



At the time we are writing, Professor 

 Barlow, of Woolwich, informs us, that 

 he has completed a fluid object-glass, 

 six inches in diameter, on a principle 

 different from Dr. Blair's, and which 

 allows him to reduce the length of the 

 telescope from one-third to one-half, 

 without diminishing its focal powerf. 



CHAPTER XI. Physical Properties of 



the Prismatic Spectrum Its Heating 



Power Invisible Rays Deoxidat- 



ing Power Chemical Rays Mag- 



netising Power Illuminating Power 



Lines across the Spectrum. 



THE physical properties of the prisma- 



tic spectrum, or, more accurately, of 



the simple rays composing white light, 



have of late years been studied by the 



most eminent philosophers, and merit 



our particular notice. 



Heating power of the spectrum. 

 From the slightest examination of the 

 prismatic spectrum, formed by refrac- 

 tion of the sun's rays, it is evident, that 

 the yellow is the most luminous of all 

 the coloured spaces ; and that the degree 

 of light diminishes both towards the red 

 and the violet extremities. Hence it was 

 naturally supposed, that there would be 

 most heat in the yellow rays, less in the 

 red, and least in the violet. Dr. Her- 

 schel, however, found, that the heat in- 

 creased from the violet to the red end of 

 the spectrum, the heat of the orange 

 being greater than that of the yellow, 

 and the heat of the red greater than 

 thiit of the other colours ; but, upon 

 placing his thermometer beyond the 

 red rays, and in the dark, he was 'sur- 

 prised to observe that the mercury still 

 rose ; and upon repeating this experi- 

 ment under a variety of circumstances, 

 lu- established the remarkable fact, that 

 the heat was the greatest at a point be- 

 yond the red extremity of the spectrum, 

 and at a spot upon which none of the 

 luminous rays at all fell. Hence he 

 concluded, that ////-/v were invisible rays 

 in the sun's light ichich had the power 



See the Edin. Jour 

 t Ibid. No. *iv. n. 333. 



>'o. viii. p. 212 



