Dr Goring on Beflecting Microscopes. 33 



the plane and the diagonal must be made larger than they are re- 

 presented, and likewise placed nearer to the concave, which, of 

 course, will cause a still larger hiatus or blot in the centre in 

 both instruments. 



Zdly, As the diagonal and plane are both nearer to the eye- 

 piece than the concave, the visual pencil, which consists of the 

 image formed by the eye-glass of the concave with a dark spot 

 in the centre, formed by the small metal, will in both cases have 

 a larger blot in it than is in the ratio of the size of the small 

 metal to the large one. Owing to these circumstances, in the 

 Amician form it is but just possible to make the image of the 

 diagonal one-third of the diameter of the concave, whereas in 

 the drawing it is only a little more than one-fourth. The large 

 plane being nearer to the concave than the diagonal, will not 

 cause an augmentation of the size of the blot, in precisely the 

 same ratio that the diagonal does ; nevertheless, the size of the 

 said black speck in the visual pencil will be larger than the 

 proportionate size of the metals, even when adapted for view- 

 ing opaque objects, will indicate. 



2)dlij, There is another circumstance to be considered, highly 

 unfavourable to the Brewsterian construction. It will be re- 

 marked that in the Amician form, the pencil of rays from the 

 object will only have to pass through the side of the tube, 

 which is very thin, at D, whereas in Sir David Brewster's it 

 must pass through the concave metal, which I have drawn no 

 thicker in proportion than in a reflecting telescope ; still it will 

 be much thicker than the tube, but I do not think that the concave 

 can be so executed. Mr Cuthbert always makes his metals 

 very thick, and also'solders a piece of brass at the back of them, 

 in order to make sure of his centering and adjustment, and 

 also to form a handle by which he may hold them comfortably. 

 and feel what he ''vas at when figuring them. Imagine a met:;l 

 of three-tenths of an inch focus, and two-tenths of aperture, 

 made as thin in proportion as that of a telescope ! How can it 

 be worked .^ People have no idea of the dehcacy of these things, 

 or how easily the figure of a metal oi ordinary thiclness is (.r- 

 stroyed. Let us suppose the artist stuck it with cement to ii 

 handle to enable him to turn and work it, and had the luck 

 afterwards to detach it without breaking it ; it is as likely as 



VOL. XXVIl. NO. LIII. JVI.V 1839. ^ 



