What sensation is it that is last extinguished, and which is possessed by a certain class of colour vision? In the Young theory it can only be the violet sensation. It is certainly not the green, and much less the red. It does not correspond, however, very well with the violet sensation shown in [Fig. 16], but more with one which should be in the blue.
In making the extinctions of light, it is quite necessary that certain precautions should be taken to avoid error. All my audience know that when going from bright daylight into a cellar, in which only a glimmer of light is admitted, but little can be seen at first, but that, as the eye “gets accustomed” to the darkness, the surroundings will begin to be seen, and after several minutes what before was blackness comes to be invested with form and detail. So it is with the extinction of light in the apparatus described. Observations carried on before the full sensibility of the eye is attained are of no value. A recorded set of observations will show this. A light of a certain character was thrown on the extinction box, to be extinguished, and the observer entered the darkened room from the full glare of daylight. The eye was placed at the eye end and kept there, and the extinctions were made one after the other till they became very fairly constant. The following is the result:—
| Times of Observation. | Readings. |
| At the commencement | 1·0 |
| After 38 sec. | 3·2 |
| After 53 sec. | 4·9 |
| After 1 min. 11 sec. | 6·9 |
| After 1 min. 44 sec. | 10·5 |
| After 2 min. 43 sec. | 17·0 |
| After 3 min. 44 sec. | 27·5 |
| After 4 min. 52 sec. | 43·0 |
| After 5 min. 59 sec. | 63·0 |
| After 6 min. 41 sec. | 78·0 |
| After 7 min. 28 sec. | 89·0 |
| After 8 min. 32 sec. | 96·0 |
| After 10 min. 46 sec. | 103·0 |
| After 12 min. | 103·0 |
(For convenience the first reading is unity; the other numbers are the inverse of the extinction value.)
The eye apparently, under the conditions in which these observations were made, was at least 100 times more sensitive to very faint light after twelve minutes than it was at the beginning, and that then concordant readings could be made. It will now be quite understood that before any serious measures can be made this interval must elapse, and also that the light, finding its way to the end of the box to illuminate the spot, should never be strong, otherwise the eye might lose its sensitiveness.
CHAPTER X.
Before considering the subject of the extinction of light by other types of colour vision, attention must be called to what has already been brought before you. The various colours of the spectrum have to be reduced to the following amounts before they suffer extinction, the orange light at D being of the value of one candle. (See appendix, [page 217], for complete tables.)
| Reduction in Millionths. | Remarks. | |
| B | 10,000 or 1/100 | approximately pure red sensation |
| C | 1,100 or 1/909 | rather more scarlet |
| D | 50 or 1/20000 | orange light |
| E | 6·5 or 1/154000 | a green chosen by Maxwell as a standard colour |
| F | 15·0 or 1/67000 | beginning of the blue |
| Blue Lithium | 85·0 or 1/11700 | a good sample of blue |
| G | 300·0 or 1/3300 | approximately pure sensation of violet. |
If we make these same colours all of the luminosity of one amyl-acetate lamp (·8 of a candle), we find that the numbers are as follows:—
| Reduction in Millionths. | |
| B | 300 |
| C | 225 |
| D | 48 |
| E | 3·3 |
| F | ·9 |
| Blue Lithium | 1·1 |
| G | 1·1 |