IV. The pupil-photometer

It is agreed on all hands that the pupil of the eye owes its size to the quality of the light; contracting and dilating according to the intensities of the light. It is not agreed, however, if indeed it has ever become the subject of debate, that itsmagnitudes may be reciprocally the measures of those intensities . There are cogent reasons for believing that they are so, and hence that an instrument which measures the pupil’s size, measures at the same time the light’s intensity. The photometer was originally constructed for the former purpose only, and indicates the diameter of the pupil in hundredths of an inch. The diameter is found by directing the instrument applied to one eye, with both eyes open, towards a sheet of white paper, or the sky; the lid of the instrument is now revolved slowly until the two white disksjust touch one another at their edges . The decimal fraction opposite the two apertures seen on the scale outside, indicates the diameter of the pupil in hundredths of an inch. On examining hourly for several consecutive weeks the light of the day reflected from a given small area of the sky, certain recurring periodicities were observed in the pupil’s magnitudes, and these are found to coincide with analogous alterations in the light’s intensity; hence it was inferred that if the pupil owes its size to the intensity of the light, it became from that very fact a measure of that intensity. It is the object of the paper to substantiate this by experiment. To use the instrument for testing light of different intensities we first set the pupil to a light of a given intensity by using a Sugg’s standard candle. This is placed at a distance ofone foot from the eye, with a white surface close behind it, in a darkened room. The diameter of the pupil is now taken under the stimulus of the candle flame, and its measure is read off on the scale of the instrument. My own pupil, when impressed with such a light, measures invariably the 0·15 inch. We now place four such candles attwo feet from the eye, when the pupil will be found to remain stationary at the same magnitude as before. These results are in strict accordance with the rule that the intensity of the illumination of any body, in the presence of a source of light, will depend upon its distance from that source, and obeys the general law of radiant forces,the intensity of the light varying inversely as the square of the distance of the luminous body . Hence, if a single candle illuminates a body at one foot, four candles at two feet are required to produce equal illumination.