The Ophthalmic Review - v. 14 Author:Unknown Author Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: C. Hess and H. Pretori (Prague). Quantitative Investigations into the Law which regulates Simultaneous Contrasts of Light and Shade. von Graefe's Arch., xl. ,... more »p. 1. The phenomena of contrast produced in the retina by two adjoining fields of different brightness are distinguished as those of simultaneous contrast when the eye remains fixed, and those of successive contrast when the eye is exposed consecutively to two fields of various brightness. An attempt to measure the apparent difference of light induced by contrast has been made already by Lehmann and by Ebbinghaus, but neither author took any precautions to keep the simultaneous contrast sufficiently separated from the consecutive, and their results are therefore not in harmony with each other. A further disadvantage of their methods was that in both cases it was not possible to change the intensities of light with sufficient rapidity and sufficient accuracy. Both these objections were overcome by Hess and Pretori who, in their researches made at the physiological laboratory of Professor Hering in Prague, dealt with the phenomena of simultaneous contrast only, to the complete exclusion of the successive contrast. The results obtained by them differ considerably from those of the two previous workers. The authors tried first to find out the changes of apparent brightness, if either the brightness of the disc or of the ground is changed. For this purpose it was necessary to construct an apparatus which permitted the lighting up within a wide range of measurable intensity, and independently of both the disc and the ground. The apparatus constructed specially for this purpose is fully described in the paper and illustrated by two sketches. It presented to the suitably placed eye of the observer a rectangular vertical w...« less