94 



REFLECTING TELESCOPE. 



magnifying poM-er four times, (or to double the lineal am- 

 plification,) preserving an equal brightness in the image; ' 

 the telescope must be made four times longer, that it may- 

 remain equally distinct *; an inconvenience, from which it 

 must be very desirable to exempt this telescope, by correct, 

 ing the figure, and, with it, the aberrations of its object 

 glasses. 

 Observation of It is not the object of this essay, to investigate any parti.. 

 Huygens that culars, in the construction of the telescope; (which would 

 dlsyroducf "' ^P'^" ^ '^^S^ ^^^^^ of inquiry ;) but to try to assist mechanical 

 indistinct contrivance in its fabrication. To this end, I think it fit 



vjsion. to add a remark, on the property of the pencils of rays, 



respecting the latitude of each of them, where they fall on 

 the pupil of the eye; first discovered by the great Mr. Huy- 

 gens ; as 1 suspect that the inconvenience he mentions, as 

 resulting from a certain breadth of the pencil, may casually 

 exceed the limit stated by him, and may admit of a practical 

 remedy. 

 The author He observes, that if the latitude or breadth of the pencils, 



thinks that the at the pupil of the eye, be very small, fso as not to exceed, 

 indistinctness -r j , ' -^ '^ ' 



arises from " ^ remember right, the sixtieth part of an inch,) the 

 fa\ihsi"the vision, by the telescope or microscope, ' will be indistinct; 

 and not in the ^° *^'^*' unless the pencils be of greater breadth than this 

 eye itself ; space, at the place of the eye, the instrument Avill be de- 

 fective: and he attributes this to something in the natural 

 conformation, or in the humours of the eye ; which, conse- 

 quently, will admit of no remedy. On this may I presume 



* A conception of this may be, perhaps, most familiarly acquired, 

 by considering, that if, of two object lenses, the aperture, and also the 

 focus of one, be twice as great as those of the other, the angles of inci- 

 dence, and refraction, of the extreme rays, which come from a very 

 remote object, and form the cones or pencils made by both, will be 

 equal; and the pencils themselves, and their aberrations, will be s'mi- 

 lar figures; all the lineal measures of that of the larger lens, being 

 double those of the other. In order, therefore, to reduce the lateral 

 error or diameter of the circle of aberrations of the former, to an 

 equality with that of the latter, while the aperture, which Is the base 

 of the pencil, remains double; this pencil iiiust be made twice more 

 narrow or acute than before. And, to effect this, its length or focus 

 (which, at first, was twice as great as that of the other pencil) must 

 be doubled; so that now it must be four times as long as the smaller 

 pencil ; i. e. the lengtlis should be as the squares of the apertures. 



ta 



