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Populäre Wissenschaftliche Vorträge

Nature volume 5, page 23 | Download Citation



THIS part of Helmholtz's essays reminds us in many respects of Tyndall's lectures—in their clear and eloquent language, eminently adapted for popular comprehension, their freedom from technical expressions, except where these are unavoidable, and in the original mode in which well-known facts are dealt with and used to illustrate profound scientific truths. The work contains six lectures, of which three are devoted to recent advances in the theory of vision, one to the correlation of the physical forces, one to the conservation of force, and the last to the objects and advances of science. In the three lectures devoted to the eye, whilst extolling its perfection as an instrument in the mode in which we use it, he points out its various defects; the blind spot, the blind lines and striæ corresponding to the vessels, its incapacity to focus equally red and violet rays, the want of uniformity in its refraction as indicated by the lines that appear to proceed from a star, &c. He discusses the various colours of the spectrum,and represents this not in the mode usually adopted of a circle with segments of various sizes corresponding to the several primary colours, but as a triangle, of which green, violet, and red occupy the angles, and blue, yellow, and purple the sides, white having an eccentric position near the yellow. Violet, which he was formerly indisposed to regard as a primary colour, he again admits, and he seems inclined to advocate, as best explaining the phenomena of colour-blindness, the views of Young: that there are special nerves for perceiving red, green, and violet rays, an opinion that is less surprising in view of Brown Sequard's conclusions in regard to the number of channels for special sensations contained in the spinal cord, and which is also supported by the remarkable specialisation shown by Helmholtz himself to occur in the branches of the auditory nerve indicated by the phenomena of certain defects of hearing. The chapters on the correlation of the physical forces and the conservation of force, subjects that are now familiar to most scientific Englishmen, are very interesting, as being, to use the German phrase, amongst the original path-breaking essays on these subjects.

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