TRANSACTIONS OP SECTION A. 52 1 



comet's spectrum which the photographs have revealed to us, are certainly charac- 

 teristic of substances containing carbon. 



In their paper ' On the spectra of the compounds of carbon ' (' Proc. Koy. Soc' 

 vol. XX. p. 494), Professors Liveing and Dewar bring forward evidence to show 

 that these two groups indicate the presence of cyanogen, and are not to be seen in 

 hydrocarbons unless nitrogen is also present. If this be the case the photograph 

 supplies us with strong evidence of the presence of nitrogen in the comet, in addi- 

 •tion to the carbon and hydrogen shown to be there by the bright groups in the 

 visible region of the spectrum. It is of great interest in connection with this re- 

 sult, now that Schiaparelli has shown us the close relationship of meteors and 

 comets, to mention the results of Professor Graham's experiments on the occluded 

 gases from the meteoric iron of Lenarto (' Proc. Roy. Soc' xv. p. 502, 1867.) 

 This iron gave nearly three times its volume of gas, consisting chiefly of hydrogen, 

 with small quantities of carbonic oxide and nitrogen. 



Professor Wright's examination of the stony meteorites shows the oxides of 

 carbon, chiefly the di-oxide, to be present in largest quantity, but he obtained also 

 a small percentage of hydrogen" and nitrogen (' Amer. Journ. Scien.' vol. x., July, 

 1875). Other kinds of meteors are known which contain hydrocarbons, even in 

 considerable quantity. It is scarcely necessary to add that, under suitable condi- 

 tions, the spectra of the gases from some meteorites will be similar to that observed 

 from the light of comets, 



Messrs. Liveing and Dewar's experiments woidd seem to show that a high tem- 

 perature must be present in the comet, if the cyanogen is fonned there, but if 

 cyanides should be found in meteorites this necessity would not exist. 



Whatever the views that may be entertained as to the forms of combination in 

 which the carbon exists, there can be no doubt whatever of the presence of carbon 

 in comets. I should mention that Mr. Lockyer regards the two bright groups seen 

 in the photograph, and the three groups in the ■\-isible spectrum, to be due to the 

 vapour of carbon at diflfereut heat-levels ('Proc. Roy. Soc' vol. xxx. p. 461). 



It is of importance to mention the strong intensity in the photograph of the 

 lines 388-3 and 3870 as compared with the continuous spectrum, and the faint 

 bright group beginning at 4220. At this part of the spectrum, therefore, the light 

 emitted by the cometary matter exceeded by many times the reflected solar light. 



On August 21, 1 attempted to obtain, with an exposure of one hour, a photo- 

 graph of the spectrum of a large comet which has appeared since Comet 'c' 1881. 

 The evening was not very favourable, and the comet was at a low altitude and not so 

 brilliant as Comet ' b.' I am not able to see on the plate more than a faint trace of 

 the brightest lines (W. L. 3883 and 3870) of the spectrum obtained from the 

 former comet. 



3. On a Prismatic Optometer. Bij Tempest Andekson, M.D., B.Sc. 



It is well known that in the normal eye, with its accommodation relaxed, parallel 

 rays of light, that is, those from distant objects, are brought to a focus on thu 

 retina. Rays from near objects are divergent, and if they enter such an eye they are 

 not brought to a focus on the retina, but would be at some point behind it.' In 

 order that they may be so brought to a focus and form a distinct image on the 

 retina, an eSort of accommodation is necessary. This is performed by a small 

 muscle called the ciliary muscle, inside the eyeball, the ultimate effect of whose 

 contraction is an alteration in the shape and perhaps the condition of the lens, 

 which causes the rays to be more strongly refracted, and brings them to a focus on 

 the retina. The effect is in fact the same as if a convex lens were added to the 

 optical system of the eye. As age advances, the muscle and lens become stiflei: 

 and work with difiiculty. They are relieved of part of their work by putting a 

 convex glass in front of the eye. Hypermetropia is a condition in which the axis 

 of the eyeball is too short, compared with the refracting power of the lens. In it 

 an eff'ort of accommodation is necessary to see even distant objects clearly, and a 

 still stronger eff'ort to see near objects. A person sutfering from it requires convex 

 glasses. When both eyes are used together, the optic axes of both are directed to 



