ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 309 



some comments on the identity of the earth called X by M. Soret, holnaium 

 by M. Cleve, and philippium by MM. Delafontaine and Marignac. 



With the view to ascertain whether the centre of the eye is trans- 

 pai'ent to the rays of extreme refrangibility of the induction spark, M. 

 Soret has made experiments on the eye of the bullock, the calf, and the 

 sheep. He finds that the limit of transparency for the aqueous and 

 vitreoas humours, with a thickness of one centimetre, is the ray U (/\ = 

 294-8, Cornu) ; with a thickness of 2-3 millimetres 16-20 (cd) are inter- 

 cepted, but 22-24 are transmitted: i.e. there is an intermediate absorption 

 or absorption-band. A diagram is given showing the maximum trans- 

 parency for different thicknesses.' As it appeared probable that the 

 absorption was due to albuminoids contained in the humours, the 

 curve due to white of egg was examined ; it is shown in the dia- 

 gram. These two curves present considerable analogy in form, but the 

 absorption-band of the latter is displaced in the direction of the less 

 refrangible end of the spectrum. Defibrinized blood also gives an 

 absorption-band similar to that of white of egg. The cornea and the 

 crystalline are more absorbent than the aqueous and vitreous humours. 

 The thinnest possible slice of the crystalline cuts off the rays of greatest 

 refrangibility. The curve due to the crystalline diluted with water 

 approached more nearly to that of white of egg than to that of aqueous 

 humour. The eye of living man is certainly more transparent for these 

 rays ; but this is due in all probability to its smaller dimensions. The 

 comparison is also made more difficult by the rapidity with which the 

 tissues alter after death. 



In any case it appears probable that the eye as a whole absorbs all the 

 rays more refrangible than U, i.e. the most refrangible ray of the fluores- 

 cent solar spectrum. 



M. Cornu, when determining the wave-lengths of the solar spectrum, 

 in order to construct a map, made some object-glasses which were 

 achromatized by using a converging lens of quartz, and a diverging lens 

 of Iceland spa.- There is not, however, the proper relation between 

 the dispersion of these two substances to give a very perfect achro- 

 matism, and in addition Iceland spa absorbs somewhat energetically 

 the most refrangible vnjs. M. Cornu discovered a substance at least 

 as transparent as quartz, and which has a law of dispersion so well in 

 harmony with that of quartz that we are enabled to obtain a system 

 of lenses of which the achromatism is nearly perfect. This substance 

 is a colourless variety of fluor spa from Switzerland. With this 

 arrangement he obtained, on one plate, with satisfactory sharpness 

 of definition, the spectrum of all the photographic rays from the 

 three blue rays of zinc to the ray No. 32 of aluminium.^ It may be 

 pointed out that the great transparency of fluor-spa for the rays of 

 highest refrangibility had previously been referred to by Professors 

 Stokes, Miller, Hartley, and Huntington. As it crystallises in the 

 tesseral system it might probably be used with advantage in special 

 researches to avoid double refraction. 



By means of the apparatus described above, M. Cornu has carried out 

 some important investigations regarding the limit of the ultra-violet rays 

 of the solar spectrum at different elevations.'' 



He finds that the limit of the solar spectrum varies with the state 



' Comptes Rendvs, t. Ixxxviii. p. 1012. ' Ibid. t. Ixxxvi. 



' Arch, (leg Sc. 2>Jiys. et nat. t. ii. 



* Cumjytes Rendus, t. Ixxxviii. pp. 1101 and 1286. 



