166 Proceedings of the British Association. 



this subject by an accidental discussion on the point whether or 

 not Aristotle, in explaining the circularity of images formed by 

 quadrilateral apertures, employed the appropriate idea when he 

 said that those images are to a certain extent quadrilateral, but 

 appear circular, because the eye is unable to recognize faint im- 

 pressions of light. Prof, Whewell, in his History of the Induc- 

 tive Sciences, had distinctly stated that Aristotle had not used 

 the appropriate idea, and that the question was entirely a geo- 

 metrical one, the appropriate idea beirjg the rectilinear nature of 

 light. Having been accidentally led to consider the subject, he 

 (Sir David) had determined in a simple manner, the form of the 

 aperture at all distances, had been led to take the same view of 

 the subject with Aristotle, who seemed to have employed the 

 appropriate idea. 



Papers by Sir D. Brewster were also read on Crystalline Re- 

 flection ; and on the Existence of a New Neutral Point and two 

 Secondary Neutral Points. 



On the improvement of the Telescope ; by Mr. Fox Talbot. 

 Mr. T. said that this subject occurred to him about two years 

 ago. when the Earl of Ross (then Lord Oxmantoun) was making 

 much larger specula for reflecting telescopes than had ever been 

 obtained before : and he thought, if once we had a very large 

 and perfect speculum, it might be possible to multiply copies of 

 it by galvanic means. He had observed, that if an electrotype 

 cast were taken from a perfectly polished surface, the cast was 

 also perfectly polished ; so that no defect of form from this cause 

 could have an injurious effect on the speculum. The great and 

 obvious defect was, that electrotypes were in copper, which 

 reflects but little light. He mentioned these ideas to Professor 

 Wheatstone, who said the same had occurred to him : and he 

 showed him a paper which he had drawn up some few months 

 before, and in which he suggested the taking galvano-plastic 

 casts of specula in platina, palladium, silver or nickel, and for 



especial purposes gilding the copper ; taking care that the two 



precipitations adhered well to each other. So that, said Mr. T., 

 the idea had suggested itself independently to both of them ; but 

 on comparing notes, they found differences. Though it had oc- 

 curred to Mr. T. to precipitate white metals, yet he did not think 

 that platina would have a sufficiently beautiful white metallic 

 polish. Prof. Wheatstone had, however, made choice of platina : 



\ 



