8. Graber had found that when animals are put into a trough covered half with blue and half with red glass, those that are “fond” of light go under the blue, those that are “fond” of darkness go under the red glass. The writer pointed out that this result should be expected on the basis of his theory of heliotropism, if the assumption be correct that the red light is considerably less efficient than light which goes through blue glass (such glass also allows green rays to go through). The botanists had already shown that red glass is impermeable for the rays which cause heliotropic reactions of plants, and the writer was able to show the same for the heliotropic reactions of animals. Red glass acts, therefore, almost like an opaque body for these animals. A closer examination of the most efficient rays for the heliotropic reactions of different organisms has revealed the fact that for some organisms a region in the blue λ = 460-490 µµ, for others a region in the yellowish-green, near about λ = 520-530 µµ is the most efficient.[236] For many plants and for some animals, like _Eudendrium_ and the larvæ of the worm _Arenicola_, a region in the blue is most efficient; for certain, if not most, animals a region in the yellow-green is most efficient. Among unicellular green algæ, _Chlamydomonas_, has its maximal efficiency in the yellowish-green and _Euglena_ in the blue. According to observations by Mast, some green unicellular organisms like _Pandorina_, _Eudorina_, and _Spondylomorum_ seem to behave more like _Chlamydomonas_, while certain others behave more like _Euglena_.[237] Wasteneys and the writer suggested that there are two groups of heliotropic substances, one with a maximum of photosensitiveness in the blue, the other in the yellowish-green; and that the latter group may or may not be related or identical with the visual purple which is most rapidly bleached by light of a wave length near λ = 520-530 µµ. [236] Loeb, J., and Maxwell, S. S., _Univ. Cal. Pub._, 1910, _Physiol._, iii., 195; Loeb and Wasteneys, _Proc. Nat. Acad. Sc._, 1915, i., 44; _Science_, 1915, xli., 328; _Jour. Exper. Zoöl._, 1915, xix., 23; 1916, xx., 217. [237] Mast, S. O., _Proc. Nat. Acad. Sc._, 1915, i., 622.