NOTES AND MEMOBANDA. 185 



power would be very easily determined, and there could be no dispute 

 as to it. Virtual images bave, nevertheless, quite as measurable a 

 size as real images, and have, like the latter, a determinate place in 

 space. 



We must not, then, gratuitously suppose that the eye constantly 

 refers them to the distance of distinct vision, because, first of all, such a 

 distance does not exist for normal eyes, and that, if even it did exist, 

 it would not be of any use for the measurement of the amplification ; 

 since each observer, and the same observer every time that he re- 

 focusses an image, places it, or may place it, at a different distance. 



It is sufiicient, to jjrove tbis, to make several people focus an 

 image, and examine its distance every time, by means of a megameter* 

 a little astronomical telescope with graduated draw-tube and micro- 

 meter eye-piece. It is thus found that nearly all the focussings give 

 different distances. 



The megameter enables us, besides, to measure, in every case, the 

 actual size of the image, by referring it, by the micrometer eye-piece, 

 to a divided scale, looked at directly through the megameter, of which 

 the focus has not been changed. The image once measured, it only 

 remains to divide it by the size of the object in order to have the 

 amplification. 



The camera lucida and the process of double vision (" double vue ") 

 also give the means of measuring the amplification, because the eye is 

 a pretty good judge of the distance of images, and consequently of 

 their size, when it can compare them to different objects whose place 

 is exactly determined (pencil, paper, divided scale, &c.). In having 

 recourse to these processes of measurjment, we recognize that in- 

 struments with virtual images give all the amplifications possible, from 

 a minimum up to infinity, each corresponding to a different distance 

 of the image. 



It is therefore inexact to say that such and such a lens, or Micro- 

 scope, magnifies the image of objects a certain number of times, unless 

 we add at what distance such an image ought to be for the indicated 

 amplification to be realized. 



The magnifying power of different instruments could be exactly 

 defined, by measuring for each of them the amplification produced at 

 a fixed distance — a decimetre, for example — because all other amplifi- 

 cations could be deduced from that, with sufficient exactitude, by a 

 simple proportion. 



What has led to the supposition that virtual images (in the Micro- 

 scope especially) were constantly referred to the same distance (the 

 distance of distinct vision), is probably the fact that, in spite of the 

 enormous variation of distance and size, which vii'tual images, given 

 by optical instruments, undergo, they always subtend in the eye nearly 

 the same augle,| do not vary sensibly in brilliancy, neither lose nor 



* See, on the measurement of map;nifying powers and the use of tlie mega- 

 meter, 'Monitore toscano,' 20 August, 1861 ; 'Memorie dtUa R. Accademia delle 

 Scienze di Torino,' vol. xxiii. pp. 455-4(35; 'Nuovo Cimento,' vol. xvii. p. 177. 



t The method employed by astronomers to measure the amjilification, gives 

 accurate results, in consequence of the almost absolute invariability of the angle 

 subtended by the image. 



