248 Proceedings of the Royal Irish Academy. 



XLI, — Ox Eecext Eeseaeches eespecxdj-g the Mixmuir Visible ix 



THE lIlCEOSCOPE. By C. E. BUEIOX. 

 [Eead, May 26, 1879.] 

 IxvESTiGATioxs for the cleterraination of the magnitude referred to in 

 the title of this paper have been recently made by Dr. Eoyston Pigott, 

 and by Professor Abbe of Jena. Dr. Pigott has accumulated a num- 

 ber of data relating to the separability of details in the images of 

 distant objects formed by any combination of lenses which is both 

 aplanatic and achi'omatic for the conjugate focal distances employed, 

 and also of short focus, when those images are viewed by a similarly 

 corrected compound microscope, the optic axes of both systems being 

 carefully adju.5tcd to coincidence. 



It is plain that if we assume that the light proceeding fi'om the 

 distant object to the focus of the image forming combination, which 

 combination we will designate as A, pursues its course in strict 

 accordance with the law of refraction, and with it alone, that we can 

 very readily detemiine the linear magnitude of any detail in the image 

 found by A, fi'om the magnitude of the corresponding detail in the 

 object used (meshes of gauze, window bars, &c.), the rate of object to 

 image being equal to the ratio of the corresponding conjugate focal 

 distances respectively. Dr. Pigott has published measurements which, 

 upon the assumption just stated, would prove that linear magnitudes 

 of less than totjto-o o-th of an inch could be distinctly discerned, and 

 yet further, that lines could be separately distinguished when their 

 reduced interval was approximately equal to i.uo\),ooo th of an inch. 



I have repeated Dr. Pigott's experiments with some slight modi- 

 fications, and have found that with two opposed objectives the equi- 

 valent foci of which were respectively ith and ^th of an inch, that 

 the image of a fine line at a considerable distance was still visible 

 when reduced to a magnitude equal, on the above supposition of a 

 strictly geometrical reduction, to over a hundred and forty thousandth 

 of an inch. Using a iVth by Eoss, and TVth by Hartnack, as the 

 image forming and examining objectives, the fine Hne used as object 

 was visible as a geometrical magnitude = a two hundi'ed and eighty 

 thousandth of an inch, and appeared sensibly as distinct as when viewed 

 under the same visual angle by the naked eye. 



The deductions just made as to the linear magnitude of the images 

 of the distant fine line are, however, shown to be untenable when we 

 consider that we have no means, as far as is known at present, of ascer- 

 taining the form and dimensions of the details of an image produced 

 in the manner above described, other than the optical examination of 

 it with the aid of a system of lenses which would produce a precisely 

 similar effect on the rays from the object if placed in a similar posi- 

 tion to that occupied by the image forming system A. Let the 

 examining system be designated B. Then the rays which have been 

 converged by A to any area in the common focal plane of the two ob- 

 jectives Avill diverge again symmetrically from that area to B, and 



