Retinal insensibility to ultra-violet and infra-red rays / by L. Webster Fox and Geo. M. Gould.

  • Fox, Lawrance Webster, 1853-1931.
Date:
1886
Catalogue details

Licence: Public Domain Mark

Credit: Retinal insensibility to ultra-violet and infra-red rays / by L. Webster Fox and Geo. M. Gould. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    have been studied. The figures 1, 2, 3, etc., give the wave-length in microns, (p), or one-hundred thousandths of a millimetre; the heights of the curved lines give the relative values in heat units, or the relative absolute energies of the different spectra com- pared. The visible and invisible solar spectrum is the first to the left and extends from wave-lengths of a size of about one-third of a micron to those nearly three in length. The other curves represent the vibrations proceeding from terrestrial bodies heated to the degrees indicated. The first and most interesting fact that appears from this extremely condensed representation is the discovery it shows of the extension of our knowledge of wave- lengths into regions hitherto unknown. We now have knowl- edge of undulations eight times the lengths known to Newton; and this is due entirely to Prof. Langley and his marvelous bo- lometer, by whose aid he measures a degree of disturbance less than the tottowo^po part, a delicacy of estimation so utterly beyond comprehension as to seem fabulous. The approximate estimate of minimum value assignable to the longest wave recognized by the bolometer in any heat from a rock-salt prism is given as 0.030 (units of a mm.) whilst the length of the short- est sound-wave corresponding to the highest musical note per- ceptible by the human ear (about 48,000 vibrations per second) is 14.0. So that, though the gulf between the shortest serial un- dulation and the longest known ethereal is still a wide one, it is far less than before, and practically inconsiderable and uninter- esting, except to the student of physical geography. The smallest waves that have been measured ai-e the extreme rays from aluminum in the induction spark, estimated at 0,185 //.. Of the solar radiations, the shortest that penetrates our atmos- phere is 0.29 p. The shortest visible to the naked eye is 0.35 , though by the aid of the fluorescent eye-piece of Soret, by pho- tography and quartz trains, vision has of late been greatly extend- ed. The longest vaves to which the retina responds are about 0.81 p, and the longest solar vibrations that penetrate our atmosphere are 2.7 p. Thus we see that the range of vision (from .35 p. to .81 p) is a little over one octave. All this is roughly shown in the figure (I) referred to. Let us now spread the solar spectrum out so to study it more exactly. This is done in Fig. II: