458 Scientific Intelligence. [June 



Article X. 



SCIENTIFIC INTELLIGENCE. 



I.' — Ashmolean Society of Oxford. 



The first memoir printed by this Society is entitled, " On the 

 Achromatism of the Eye. By the Rev. Baden Powell, A.M." &c. 

 It is well known that when rays of light are inflected by a lens 

 they undergo a deviation, by which they are prevented from con- 

 centrating in the same point or focus. This aberration gives origin 

 to the production of colour at the foci of lenses, and constituted a 

 great imperfection in refracting telescopes until the discovery was 

 made, that a compensation for the deviation of the rays of light might 

 be effected by employing compound lenses. Those telescopes in 

 which this improvement was introduced were termed achromatic. 

 Now, as there appears no compensation in the eye for this aberration, 

 it is natural to inquire into the reason of our seeing objects without 

 prismatic colour. Such is the question which Professor Powell 

 undertakes to investigate in the present paper. He presents us first 

 with the opinions of various philosophers in reference to the subject, 

 and then supplies us with inferences drawn from his own experi- 

 ments. D'Alembert admitted the want of achromatism in the eye, 

 but considered the aberration very small. Euler held an opposite 

 opinion, and Dr. Maskelyne refuted the arguments of Euler. Dr. 

 Wells has observed that the eye has no principle of achromatism, and 

 Sir David Brewster says that " no provision is made in the human 

 eye for the correction of colour, because the deviation of the differently 

 coloured rays is too small to produce indistinctness of vision." Mr. 

 Coddington states that the eye, when employed in its natural and 

 proper manner, is achromatic. The fact is, that we see objects with- 

 out the slightest degree of prismatic colour or indistinctness. The 

 question then is, how can this be reconciled with theoretical consi- 

 derations ? Mr. Powell, by ingenious calculations, has inferred that 

 in such a combination as the eye, exact achromatism is perfectly 

 possible in theory, and that the principle of its achromatism, although 

 not effective in obtique excentrical rays, may be in general achromatic 

 for direct rays. He gives the results of a series of experiments, 

 in which he has ls endeavoured to ascertain directly the actual 

 prismatic dispersions of the crystalline and vitreous humours, by 

 measuring micrometically the separation of the different parts of the 

 spectrum of a line of light produced by looking through a prism 

 formed of each medium, from the eye of an ox, between inclined 

 glass plates." From which he concludes " that the media of the eye 

 have as nearly as possible those dispersive powers and relations of 

 indices for the different rays, which theory requires for producing 

 achromatism by means of a single lens, when the focus is formed in 

 a dense medium." 



February 13, 1835 — Mr. Twiss of University college exhibited 

 some specimens of the papyrus from Syracuse, both in its natural 

 and manufactured state. He read some observations upon it, describ- 



