Now, if we allow this prismatic spectrum (b. Fig. 3.) to fall upon any surface (as at c.) prepared with a sensitive photographic compound, we shall find that the chemical effect produced bears no relation to the intensity of the light of any particular colored ray, but that, on the contrary, it is dispersed over the largest portion of the spectrum, being most energetic in the least luminous rays, and ever active over an extensive space, where no traces of light can be detected. Fig. 4, will give the student a better idea of this principle. It is a copy of the kind of impression which the spectrum, spoken of, would make on a piece of paper covered with a very sensitive photographic preparation. The white space a. corresponds with the most luminous, or yellow ray, (5, Fig. 3) over limits of which all chemical change is prevented. A similar action is also produced by the lower end of the red ray c; but in the upper portion, however we find a decided change (as at d). The most active chemical change, you will perceive, is produced by the rays above the yellow a; viz. 4, 3, 2 and 1 (as at b) the green (4) being the least active, and the blue (3) and violet (1) rays the most so, the action still continuing far beyond the point b which is the end of the luminous image. [Illustration: Fig. 4 (hipho_4.gif)] Suppose we wish to copy by the Daguerreotype, or Calotype process, any objects highly colored--blue, red and yellow, for instance predominating--the last of course reflects the most light, the blue the least; but the rays from the blue surface will make the most intense impression, whilst the red radiations are working very slowly, and the yellow remains entirely inactive. This accounts for the difficulty experienced in copying bright green foliage, or warmly colored portraits; a large portion of the yellow and red rays entering into the composition of both--and the imperfections of a Daguerreotype portrait of a person with a freckled face depends upon the same cause.