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