Optical Inventions, hi/ Prof. Aniici. 401 



pectations. One of them, less than an inch in length, and half 

 an inch in breadth, composed of little prisms of French glass, 

 with angles of 45°, affords so much distinctness and precision 

 in the outlines of the image, that it exceeds in its effect the 

 most perfect achromatic opera glasses." 



The construction of this extraordinary machine he has not 

 more particularly described : he has, however, explained the 

 general principle upon which it depends, and although it may 

 be apprehended that it can never be applied to instruments of 

 material importance, it really appears to have afforded him an 

 elegant little plaything. 



" Supposing, that through a prism, having its axis in a verti- 

 cal direction, we look at a small square object, having one of 

 its sides also vertical, it is obvious, that if we turn the edge of 

 the prism so as to incline it towards the object, the image will 

 become an oblong rectangle, instead of a square. If we then 

 take a second prism of the same substance, and place it behind 

 the former, with its axis horizontal, and turn it until it produces 

 an equal deviation, the image will manifestly be prolonged in a 

 vertical direction, and will again become a square, magnified, 

 but still coloured. Now, since a coloured spectrum of a given 

 extent may be produced in two different ways, that is, either by 

 turning round its axis a prism with a small angle, in order to 

 increase its refractive effect, or by making a prism of the same 

 substance with a larger angle, it will be easy, without recurring 

 to the first method, which would produce a distortion of the 

 image, to determine the angle to be given to a third prism, in 

 order that its least refraction may produce a spectrum of equal 

 extent to that which is formed by the two combined prisms. If 

 then we place this third prism behind the two former, in such a 

 mamier that its refraction may be in the direction of the diago- 

 nal of the square, it will correct the dispersion of the colours, 

 without distorting the object, which will of course still remain 

 magnified ; so that the system of these three prisms alone will 

 constitute an achromatic telescope consisting of a single refrac- 

 tive substance only." 



CoHlMcrroN uf Art. v. No. VI. 1'. 370, Line; H (roiii the IioHdiii, for "iiiui 

 riiiigc(|U('iitly lo the liinKnelic axis of the earth," read " and ci)ii.s((|uciilly to 

 that ol the ili|>i)inK.ncculc." 



