OpliitU fnvc/tlhms, bij Prof. Aniici. 399 



Kratzeiistein and Euler : but it was demonstrated by Boscovich 

 that " an iconantidiptic telescope with three achromatic object 

 glasses produces a much less effect than a common telescope of 

 half the length," so tliat the advantage gained by the double 

 velocity, with which the two images approach each other, is no- 

 thing more than might be obtained from a common telescope 

 with twice or more than twice the magnifying power, as it might 

 easily be made. He also showed that the telescope must fail in 

 its proposed object of superseding the necessity of micrometer 

 wires, for that the images could never be made to meet in the 

 axis without having the assistance of a wire to guide the direc- 

 tion of their motion. 



Mr. Amici observes that the optical difficulty might be 

 avoided, by employing a reflecting instead of a refracting tele- 

 scope, and by combining the constructions of Gregory and Cas- 

 segrain in the same instrument ; but that the weight of such a 

 telescope would render it inconvenient for astronomical obser- 

 vations. Still, however, the wires would be required, and the 

 advantage of viewing very faint stars, without the danger of ex- 

 tinguishing their light by the illumination, would be lost. He 

 therefore proposes to obtain a double image by reflecting half 

 the rays, so that it may be formed in a simply inverted and 

 wholly reversed position ; and that if the plane of reflection be 

 situated, for example, in that of the meridian, the images of all 

 the stars in the field would meet each other at the true moment 

 of their transit, while, in the construction of Jeaurat, none of 

 them could coincide unless they were precisely in the axis of 

 the tube. For this purpose he places, " in the focus of the eye- 

 piece next to the object glass, a small rectangular isosceles 

 prism of glass, the plane opposite to the right angle passing 

 through the axis of the telescope, and its edges being perpendi- 

 cular to it, so as to intercept half of the pencils of rays belonging 

 to each point of tlie object, and to form with them a reflected 

 image" depending on the internal reflection of the base of the 

 prism. The instrument in this form appears to be peculiarly 

 adapted to the observation of transits, though it may possibly 

 retjuire some modification both in the angle of the prism, and in 

 its place with regard to the focus of the telescope. 



