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Another criticism I would make is in reference to the printed slip that you would use in this instrument. After you have used it for a while, you get to read it more easily than at first. If you are going to change that printed slip every time you use the instrument, you may change the character of the surface; you may have a surface not so white, or a little different, and you cannot be sure you are getting the standard surface. Every one knows that if he has once read a thing he can read it much more easily if he has to read it again, and knows what is there. The effect of imagination comes inthat is action which has to be counted on. It is a question in my mind, with an instrument of this kind laid aside and not used for a time, whether the paper will retain its integrity as a surface for receiving and diffusing light. We know that it is darkened by age. I think it would certainly be required that we should use some material like porcelain, like a watch face, absolutely permanent, if we employed the method at all.

There is another question. Is it true that with different individuals the iris opens or responds equally; do the eyes have the same aperture to receive light; and are the retinal surfaces of the different eyes equally sensitive? I have my doubts, so far as my observation goes. In the eye of a child, the iris opens wide; but with an older person, the eye is not so flexible; in fact, it may finally become partially fixed. It will not give the same aperture of illumination; the diaphragm is smaller, and does not respond so easily. In certain forms of incipient disease, the first indication is that the iris does not respond easily. A slight injury to the eye may cause a difference in sensibility of the eyes. My eyes happen

to be in that condition now; and it brings the matter forcibly to my mind. There may be a lack of sensitiveness for the time being in one of the eyes. Again, I think our eyesight varies even as we are looking. Everybody who has opened his eyes in a dark room will be able to distinguish objects for a time, and then a black curtain, as it were, passes over the sight, and then the objects come out again. What percentage of difference that would make in reading anything or seeing anything that was at all illuminated with the eye, I do not know. That is simply something which I put out as a suggestion. But it is a matter of my personal observation that if I am looking in a much darkened room I see all the objects for a little while, they disappear, and then I see them again, as if the sensitive material of the eye were renewed in gushes. There is some peculiar connection of the sensorium, which, as it were, brings on a wave of sensibility, wave of sensibility, then wipes it out, and then brings on another wave.

Then comes the question of color-blindness in people. I have met a number of people subject to color-blindness-not sensitive to the red rays-some more sensitive than others. It is well known that the power of vision with the higher rays, the ultraviolet rays, is very different in different observers; some can see the lavender in the spectrum, others. scarcely at all. It happens that at the extreme violet part of the spectrum there is not a great amount of useful illumination, but still there is some, and it might make a difference in apparent illumination in such cases. I simply bring these points forward as indicating that we may find it necessary, before adopting a method of the kind used in the instrument before us, to compare the results of a large number

of observations to find out whether there was a difference to be noted, or whether we could rely upon a fair average result. Ten per cent out would not bother us very much, and perhaps all these errors combined might, if we used due precautions, be included within the ten per cent. Then the instrument would undoubtedly have my unqualified endorsement.

MR. AYER: The adoption of an instrument of this character would seem to me, as expressed by the other speakers, to be a thing that should receive a great deal of consideration. Aside from the different results that might accrue from the difference in the eyes of different people, is the integrity of the persons using it. The point made by Professor Thomson about the familiarity with the letters, where imagination comes into play, is a very important one. We had a case of this kind where there were produced a number of sheets of paper with varying sizes of letters printed on the sheets. A committee was selected, who remained with their backs toward the light, so as to avoid affecting the vision by looking at the lamps, and each one would walk a definite distance away and then endeavor to read a certain number of lines aloud. We found variable results. One man would read line number two at 200 feet; another, line number three at the same distance, and so forth; but as we proceeded we found a change. We found that the man who at first could only read line number two 200 feet away could read line number three, the latter being a finer print. The men, I think, were naturally unbiased in the matter, but they gradually became familiar with the print. To overcome this familiarity, on another occasion, the chairman of the committee had a series of slips with a line of Volapük, a

half-dozen words on each line, and it was practically impossible to memorize it. After an evening's work of several hours, we found the same persons generally read the same lines at the same distance. Where other conditions remained unchanged, the characters could not be so easily memorized; giving abundant evidence to my mind that the imagination is an important element.

MR. KENNELLY: The first point raised by Mr. Haskins is the question as to the relative sifting power of the translucent screen for various colors of light; but it seems to me and I think it will seem to you on reflection-that that difficulty is entirely eliminated by the very method employed in calibration. If we have any standard of light-if we have any basis for measuring light-it is a standard of intensity; that basis may be a standard candle or a Hefner Alteneck lamp. Whatever these values may be, they are standard, and are recognized as such. If with a definite standard you place the instrument at measured distances from that standard, you have a definite known illumination; and if you calibrate and mark off the scale of the instrument to that illumination, you have eliminated for that character of light all possible questions as to the degree of sifting of the translucent material. It is true that the argument does not hold so soon as you change the character of the light. The illumination measured on this table by incandescent light, if an incandescent lamp had served you as a standard and a calibration had been made by an incandescent lamp, might cease to be the true illumination measured by oil or arc light; but by actual trial we have found. that the error that may be induced by the variation which must exist, owing to the absorption of the

translucent material for different colors, is too smal to be noticed. A sixteen candle-power lamp measured for its intensity by the instrument in its use as an indirect photometer, agrees with the measurement obtained by reckoning it from calibration obtained by a standard candle, within the limits of observational error in the instrument.

In regard to what Professor Thomson has so eloquently explained with reference to the unfortunate defects in our eyes, I grieve that our eyes should be open to such grave impeachment; and, in fact, I am only glad, after hearing the troubles to which we may be liable, that we have eyes that are of use for any purposes of vision, after all. No two eyes see precisely alike, of course, and it is not possible to get an instrument based on the agency of the eye to be absolutely correct; but, on the other hand, I am pleased to believe that our eyes are of some definite value, and just in so far as our eyes are perfect and normal, just in so far can we conduct measurements of illumination that may be relied upon.

In regard to the characters on the test object in the illuminometer, there is no doubt as to whether you know them or not. They are only a group of five numerals, and after you once read them, you can scarcely fail to remember what you have read. You know very well what the characters are. You simply endeavor to bring down their illumination until you cannot read them. All these errors, which are inherent in any apparatus involving the eye, have, after all, a certain limit. I will ask any gentleman present if he can tell me what the illumination upon the surface of this table is, within fifty per cent. should be surprised if any could readily guess within fifty per cent what it is.



We can measure it

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