April 3, 1885.] 



♦ KNOWLEDGE ♦ 



285 



6. Gklatixe, Mica, Talc. — A thin sheet of colourless 

 gelatine, or of the minerals mica or talc, may have lines 

 scratched upon it with a nrctllo. The slide must then be 

 rubbed over ■with lampblack, which will lodge in the 

 scratches ami produce black lines when the slide is shown 

 upon the screen. Sheets of colourless gelatine can be 

 bought for this purpose, or they may be prepared by 

 soaking the gelatine in cold water, then melting it by a 

 gentle heat, and finally pouring the fluid upon a sheet 

 of clear glass which has previously been rubbed with 

 French chalk. When dry, the gelatine can readily be 

 detached. 



The last three methods are convenient for those who 

 possess but little artistic ability, inasmuch as the trans- 

 parent substances employed can be used like a child's 

 transparent slate, the slide being placed over the diagram 

 ■which it is desired to copy, and the lines traced upon it. 



{To le continued.) 



IS THE DIAMETER OF THE PUPIL OF 

 THE EYE AN EQUIVALENT OF 

 THE LIGHT'S INTENSITY? 

 By John Gorham, M.R.C.S. Eng. 



(From a paper read before the Eoj-al Society in November, 188-1.) 



IN this paper it is proposed to inquire whether the 

 human eye is endowed with the power of measuring 

 the varying intensities of light which fall upon the retina. 

 To the physiologist who has investigated this confessedly 

 most difficult department of optics such an inquiry will not 

 be without interest, since the path in this direction is com- 

 paratively an unfrequented one. Little has yet been done, 

 and that little, so far from strengthening our conviction as 

 to the ability of the eye to indicate light of different 

 intensities, has resolved itself into a deliberately announced 

 persuasion that not only is this organ but little capable of 

 judging of the relative strength of difTerent illuminations, 

 bnt that from its very nature and its incessant movements 

 it is actually incapacitated for the task. See what a high 

 authority says on the subject : (Sir John Herschel, Art. 

 Light, Cyclop. Metrop.) : — 



We know of no instrument, no contrivance, as yet, by which 

 light alone (as such) can be made to produce mechanical motion 

 BO as to mark a point upon a scale or in any way to {^ve a correct 

 reading off of ita intensity or quantity at any moment. This 

 obliges us to refer all our estimations of the de^ees of brightness 

 at once to our organs of vision, and to judge of their amount by the 

 impression they produce immediately on our sense of sight. But 

 the eye, though sensible to an astonishing range of different degrees 

 of illumination, is (partly on that very account) but little capable 

 of judging of their relative strength, or even of recognising their 

 identity when presented at intervals of time, especially at distant 

 intervals. In this manner the judgment of the eye is as little to be 

 depended on for a measure of light, as that of the hand would be 

 for the weight of a body casually presented. This uncertainty, too, 

 is increased by the nature of the organ itself, which is in a con- 

 stant state of fluctuation : the opening of the pupil which admits 

 the light continually expanding and contracting by the stimulus of 

 the light itself, and the sensibility of the nerves which feel the im- 

 pression varying at every instant. 



But yet more, when two unequally illuminated objects (surfaces of 

 white paper, for instance) are presented at once to the sight, though 

 we pronounce immediately on the existence of a difference, and see 

 that one is brighter than the other, we are quite unable to say 

 what is the proportion between them. Illuminate half a sheet of 

 paper by the light of one candle, and the other half by that of 

 several, the difference will be evident. But if ten different persons 

 are desired, from their appearance only, to guess at the number of 

 candles shining on each, the probability is that no two will agree. 

 Nay, even the same person, at different times, will form different 



judgments. This throws additional difficulty in the way of photo- 

 metrical ostimntions, imd would soem to rcmler this one of tho- 

 most di'licatu and dillicult departments of optics. 



Now, -with due deference to tho reason assigned by this 

 eminent philosopher for tho eyes' inability to recognise 

 dilTerent degrees of illumination, one of which, by the way, 

 constitutes in itself an important item of its efliciency in 

 this direction — e t/., "the. ccmthmal expansion of the jnipil 

 by the stimulus of the light," we remark that that part of 

 the eye which is exclusively engaged in recognising these 

 illuminations, and measuring them, so to speak, from 

 morning till evening, all the year round, was entirely 

 ignored in his speculations. Vision, bo it observed, is per- 

 formed by the combined agency of two distinct sets of 

 stimuli, the one to superintend and control the brilliancy 

 of tho light -which enters the eye, the other to assist in 

 utilising tho light thus provided ; the former is a 

 reflex or instinctive act ; the latter a voluntary 

 one. Now, although by this last wo become cognisant of 

 the general effects of lights of different brilliancies — such 

 as a bright day as opposed to a dull one, or the dim twilight 

 as compared with the intense light of midday— we cannot 

 by any voluntary act judge of minute differences or tran- 

 sitions in these intensities, any more than we can by the 

 same voluntary act judge of the measure of an inch in 

 length. These operations of judgment and exertion are so 

 various that they cannot be relied on. It is at this crisis 

 that the pupil interposes, and furnishes, by its instinctive 

 powers, just that information as to the light's intensity 

 which it was impossible to obtain by any voluntary effort 

 of our own. 



When the light falls upon the eye, therefore, the pupil 

 instinctively assumes a given magnitude, which is always 

 the same in the same person, under the .stimulus of a light 

 of the same intensity. Over this magnitude we are con- 

 scious of exercising no voluntary control. 



We may conclude, in brief, that the pupil measures the 

 light for the retina, the retina uses that light for visionv 

 and -we should have experienced the same difficulty with- 

 Sir John Herschel, in judging of the relative strength of 

 different lights, if we had chosen, as he did, the retina 

 with the mere exercise of its voluntary power, to tha 

 exclusion of an instinctive pupil, by which alone the feat 

 can be accomplished. 



* * * * * # : r 



[Here in the MS. appears a description and illustration 

 of Mr. Gorham's instrument. Inasmuch, however, as this 

 follows closely on the lines of our own article on p. 465 o£ 

 the last volume of Knowledge, the reader is requested at 

 this point to turn back and reperuse the account of the 

 Pupil Photometer which we there gave. — Ed ] 



{To he continued.) 



SLATE IN AMERICA. 



FEW people have any idea of the magnitude of the 

 slate industry in this country. Until a few yeara 

 since, the jiroduct of the difl'erent slate quarries in the 

 United States was quite limited. Now the total amount 

 produced, of roofing-slate alone, is about 500,000 squares 

 a year. A "square" is 100 square feet, or sufficient to 

 cover a space of 10 ft by 10 ft, when laid on the roof. It 

 covers the .same area as 1,000 shingles, and sells for from 

 .3-50 dels, to 450 dels, jier square. 



As a roofing material slate is becoming more generally 

 used, as it lasts a lifetime, is fire-proof, needs no painting, 

 and renders rain-water pure and untainted. Besides the 



