J92 A Reply to Earl Stanhope, on his Defence of 



correct figure to a convex than to a plane speculum ; and it 

 is well known to practical opticians, that the errors of one 

 spherical speculum often correct those of the other. 



If any real advantages arise from shortening the tubes of 

 reflecting telescopes, it becomes a matter of importance that 

 refracting telescopes should possess similar properties. By 

 means of the following, contrivance the tubes of refractors 

 may be so much shortened as to be only one third of the 

 focal length of the object-glass. A plane speculum CD, 

 whose diameter is two-thirds of that of the object-glass AB, 

 is so placed that CA is one-third of the focal length of AB. 

 By giving a small inclination to CD, the rays are reflected to 

 a second plane speculum EG, equal to one-third of the dia- 

 meter of AB, which again reflects the incident rays to F, 

 where the image is formed and magnified by the eye-piece. 

 In this construction, the only disadvantage is the loss of light 

 occasioned by two reflections ; but this may be obviated by 

 increasing the aperture of the object-glass, and is by no 

 means such a serious evil as that which arises in M. Burck- 

 hardt's contrivance, from the loss of such a large central 

 portion of the great speculum. 



I am, dear sir, your most obedient servant, 



D. Brewster. 



To Mr. TillocJu 



LI I. A Reply to Earl Stanhope, on his Defence of certain 

 Principles and Facts erroneously stated in his Stereotyped 

 " Principles of the Science of Tuning Instruments with 

 fixed Tones." By Mr. John Farey. 



" The difference between a man of real science, and one who has the ain- 

 bition So be thought so, is very great." — Earl Stakhof-e. 



JL h: 



To Mr. Tilloch, — Sir, 



ie truths and principles of the Mathematical Sciences 

 are not in any instance to be yielded to authority, however 

 imposing its aspect; neither should we suffer any other con- 

 siderations, long to restrain our efforts, in defending their 



just 



