140 Rev. W. Hodgson on New Applications of Prisms. 



an optical square should have been introduced, when the pur- 

 pose for which it was constructed is answered either by the 

 camera lucida or by the rectangular prism. 



There remains still one other application of the rectangular 

 prism, which is too important to be overlooked. If a prism 

 of this form be placed over the eye-piece of a compound mi- 

 croscope, or of an astronomical telescope,* so that its axis is 

 inclined to that of the microscope or telescope at an angle of 

 45°, the rays, after suffering two internal reflexions, will 

 emerge without chromatic dispersion, and in such a manner 

 as to do away entirely with the inversion produced by those 

 instruments. There is, however, a great loss of light, in 

 consequence of the obliquity at which the rays enter the 

 prism. With a view to remedy this defect, various plans 

 were tried. Two equilateral prisms cemented together, with 

 their axes perpendicular to each other, were found to erect 

 the image without loss of light, and to allow of its being 

 seen as if projected on a screen placed to receive it : but 

 there was a deviation to the right or to the left, which was 

 unsatisfactory. The regular tetrahedron also was tried, and 

 after trial rejected for a similar reason. At length the solid 

 represented in fig. 6 was adopted, and found to answer com- 

 pletely. The rays are incident upon the first surface per- 

 pendicularly, the internal reflexions are both total, the solid 

 is of a form which admits of easy adaptation to the eye- 

 pieces, the movements of the image correspond exactly with 

 those of the object, and the image may be traced upon a 

 screen placed behind it, as readily as with the camera lucida. 

 The lower surface might, if necessary, be ground into the 

 form of a lens, but this, on other accounts, is inconvenient. 



Old Beathay, IZth November 1851. 



* It is not anticipated that this mode of erecting the image will be generally 

 used by astronomers ; but the addition of such an appendage to the eye-piece 

 of an astronomical telescope of low power forms an improved graphic telescope, 

 which may be serviceable to draughtsmen. 



