oftlie Cassegrainian Telescope, &c. fttt 



The magnifying power having been determined (by expe- 

 riment) to be 188 in the Cassegrainian, and 182 in the Grego- 

 rian, the expression for the relative quantity of light becomes 



4=^ to -l~^ or as 33a to 224, being nearly as 3 to 2. 



In the first experiment, the advantages of polish were, per- 

 haps, on the side of the Cassegrainian telescope ; in the last, 

 they were much in favour of the Gregorian ; a mean therefore 

 of both results may probably be considered as approaching 

 the truth, and the light of a telescope of the Cassegrainian 

 construction, may be taken, to that of a Gregorian of the same 

 aperture and power, as about 60 to 33. 



A fact so new, naturally leads the mind to hazard a conjec- 

 ture as to the cause. In the Gregorian telescope a column 

 of light from a point of the object, is received on the large 

 mirror, and reflected in a cone of rays, the vertex of which is 

 its focus, where an image is formed. Here all these rays 

 meet in a single point, and crossing each other, fall on the 

 small concave mirror whence they are again reflected, and 

 form another image near the eye. Now, if light be supposed 

 to consist of particles of matter, is it not possible that these 

 particles, crossing in the same point, may interfere with each 

 other ? or, when thus forced within a certain distance of each 

 other, may not a power of repulsion exist, which would occa- 

 sion many of them to be dissipated ? In the Cassegrainian 

 telescope the rays reflected from the great mirror are received 

 by the small convex mirror before they arrive at their focus, 

 and are consequently reflected back without having crossed 

 as in the Gregorian. The conclusion then seems to be, that 

 wherever an image is formed, much light is lost, and this con- 



MDCCCXIII. F f 



