Mechanical Science. 199 



the imao^e strongly contrasted by the effect of a good sinj^le triple- 

 j^lass of equivalent power. When, however, only two of the object- 

 glasses are combined, the effect is very fine. Between the object-glass 

 and eye-glasses is placed one of those prisms originally invented by 

 Sir I. Newton to act as an eye-piece of his telescope, and of which 

 a description may be seen in his correspondence at the end of Dr. 

 Gregory's Optics. The utility of the introduction of this device 

 appears very questionable in an instrument already so complicated. 

 The diversion of the rays into a course at a right angle to their 

 original progress (merely to give an horizontal instead of a vertical 

 position to the body) is surely no warrant for the employment of 

 two extra refracting surfaces and one reflexion, which cannot fail to 

 have a pernicious influence on the formation of the image. An 

 horizontal position of the body is attained with the utmost facility 

 by a proper construction of the mounting, &c. Setting aside the 

 dulness of the image produced by the numerous refractions, the 

 performance of the instrument on test-objects was highly respect- 

 able and satisfactory. 



The reflector is a modification of the original construction recom- 

 mended by the Professor, who seems to have profited by the school- 

 ing he received from Dr. Goring, and now sails much closer to the 

 wind than he did. His objective metal is now two inches focus, 

 with an aperture of 1^ inch ; but half an inch is cut off for the 

 purpose of preventing the bad effect of the marginal rays, so that 

 only 1 inch of the central portion of the metal is employed ; — the 

 diameter of the diagonal mirror is also reduced to its proper stand- 

 ard, by which means the blot in the centre of the visual pencil is 

 rendered as small as possible. It may be asserted of this instru- 

 ment, that it does as much as can possibly be expected from an 

 objective part of 2 inches focus, showing many test-objects faintly, 

 and with much effort; but it is totally unable to compete with 

 deeper ones equally perfect and of the same angular opening. 

 The Professor has, in some of his instruments, reduced the focus of 

 the elliptic metal to ^ inch, and will, no doubt, gradually slide 

 into the adoption of that radical reform in his instrument, so hap- 

 pily carried into effect in this country by Dr. Goring, in conjunc- 

 tion with Mr. Cuthbert, — at least if the figuration of elliptic metals 

 of -j^(f inch focus with -^^ inch of aperture shall not surpass his 

 powers of execution. During the Professor's stay in this country 

 there was a grand field-day at his hotel, at which both his micro- 

 scopM were tried ai^ainst the Goringian modijication of the reflector, 

 the superior weight of metal of which completely beat every thing 

 opposed to it. For the honour of the Professor it must be stated, 

 that he admitted this defeat with great candour and good sense, 

 and even had some difficulty in believing in the identity of some of 

 the objects used, so differently was the ordinary apparent structure 

 developed by the English improvements on his instrument. It 

 may with safety be averred that no refractor, at least, will ever be 



