American journal of physiology . sideredis one that was firstclearly observed byOltmanns ^ in experi-ments with Volvox(globator and minor).It may essentially bestated as follows: Ifan organism is free tomove at right anglesto a parallel series oflight rays artificiallyso modified that theillumination at oneend of the course isbright and graduallyshades off to darknessat the other end, the organism again seeks its region of optimal illumination. Oltmannsrealized these conditions practically as follows (Fig. 5). Parallelrays of light fell horizontally at right angles to the long axis ofa narrow
American journal of physiology . sideredis one that was firstclearly observed byOltmanns ^ in experi-ments with Volvox(globator and minor).It may essentially bestated as follows: Ifan organism is free tomove at right anglesto a parallel series oflight rays artificiallyso modified that theillumination at oneend of the course isbright and graduallyshades off to darknessat the other end, the organism again seeks its region of optimal illumination. Oltmannsrealized these conditions practically as follows (Fig. 5). Parallelrays of light fell horizontally at right angles to the long axis ofa narrow trough of water destined to receive the organisms. Be-tween the trough and the incoming light there intervened a prismaticscreen. Thin at one end, it there let almost all the light through;but becoming gradually thicker it gradually diminished the intensityof the rays till at the opposite end the screen was opaque and inter-cepted all the light. A volvox placed in the trough was thus free to^ Oltmanns: Flora, 1892, Ixxv, p. Lvnceus Figure 5. — Observed phototactic movements of Stentorand Lynceus in aquaria seen from above and exposedto light that has passed through a prismatic screen. 472 Edivin B. Holt and Frederic S. Lee. move from light to darkness and conversely, but was obliged to movealways at right angles to the rays. Because of Oltmanns incautiousconclusion ^ that the determinative factor is not the direction inwhich the waves of light . . are propagated, but rather the locationof the optimum, that is the direction in which the intensity or con-centration varies gradually toward the optimum, his experimentshave been rather disparaged by those who believe in the efficacy ofdirection.^ And it is to be granted that some peculiarities in hisarrangement of light must have occasioned rather complicatedshadows. But this much can hardly be denied, that his rays wereperpendicular to the prism, and that his animals collected in thetrough in places opposite to points of th
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