When the spectrum is produced by prisms the intervals between these lines are not proportional to the wave-lengths, and consequently if we measure the distance of a ray in the spectrum from two of these lines, we have to resort to calculation, or to a graphically drawn curve, to ascertain its wave-length. For the purpose of experiments in colour the graphic curve from which the wave-length can immediately be read off is sufficient. The following diagram (Fig. 3) shows how this can be done. The names and range of the principal colours which are seen in the spectrum has been a matter of some controversy. Professor Rood has, however, made observations which may be accepted as correct with a moderately bright spectrum. If the spectrum be divided into 1000 parts between A in the red, and H, the limit of the violet, he makes the following table of colours. +---------------+--------------------------------+ | Scale. | Colour. | +---------------+--------------------------------+ | 0 to 149 | Red. | | 149 to 194 | Orange red. | | 194 to 210 | Orange. | | 210 to 230 | Orange yellow. | | 230 to 240 | Yellow. | | 240 to 344 | Yellow green and green yellow. | | 344 to 447 | Green and blue green. | | 447 to 495 | Azure blue. | | 495 to 806 | Blue and blue violet. | | 806 to 1000 | Violet. | +---------------+--------------------------------+ Fig. 3.--Curve for converting the Prismatic Spectrum into Wave-lengths. In the above scale (Fig. 3) A = 0, B = 74·0, C = 112·7, D = 220·3, E = 363·1, F = 493·2, G = 753·6, H = 1000. These are the main subdivisions of colour, but it must be recollected that one melts into the other. When the spectrum is very bright the colours tend to alter in hue; thus the orange becomes paler, and the yellow whiter, and the blue paler. On the other hand, if the spectrum be diminished in brightness the tendency is for the colours to change in the opposite direction. Thus the yellow almost disappears and becomes of a green hue, whilst the orange becomes redder, and the spectrum itself becomes shorter to the eye than before. Let us strictly guard ourselves, however, from the criticism that all eyes see not alike. Suffice it to say that the above table is correct for the ordinary or normal eye, and does not necessarily apply to those who have defective vision as regards colour sensation. CHAPTER III. The Visible and Invisible Parts of the Spectrum--Methods for showing the Existence of the Invisible Portions--Phosphorescence--Photography of the Dark Rays--Thermo-Electric Currents.