1882.] 



MICEOSCOPICAL JOtJENAL. 



155 



lens, telescopic or microscopic. Let 

 us see what significance attaches to the 

 proposed substitute / o" /'. It certain- 

 ly does not give us, when supple- 

 mented by the tube-length used in 

 measuring it, the angular breadth of 

 the cone of rays transmitted from ob- 

 ject-point to image-point, nor any ca- 

 pability of the lens dependent on that 

 item, unless the linear aperture of the 

 field-glass is specified in addition. 

 Even then considerable calculation 

 is needed to ascertain the very points 

 easily obtainable from / o I' . Addi- 

 tional complication arises from the 

 fact that lo" l' increases or diminishes 

 as the tube is shortened, accordingly 

 as the linear aperture of the field- 

 glass is greater or less than that of 

 the objective. But the implication 

 that the so-called telescopic angle 

 I I' and the so-called microscopic 

 angle lo" l' are constructed on a simi- 

 lar principle, or by a similar method, 

 is the most surprising feature of Mr. 

 Cox's paper. 



The image c' d' from which I o I' \% 

 derived, is said to be " bounded by 

 the marginal rays of the cone / o' I' . 



Of course, the portion of an image 

 at c' d' which is visible to an eye at o' 

 is so bounded, but there is no special 

 significance in that fact, except that 

 the partial image taken as the " tele- 

 scopic image " varies with the size of 

 the lens, though the whole image does 

 not. 



If the visible portion viz., c' d' of 

 the whole image furnishes a basis for 

 / I', then an angle intended to be 

 analogous to I o I' should be based 

 upon the partial image analogous 

 to the portion / d'. Such analo- 

 gous portion of the image formed 

 at a' b' must be smaller than the 

 visible portion of that at c' d', because 

 the eye would be moved backward as 

 far from o', as o' is from c' d' . Al- 

 though it is stated in the paper that 

 " in both cases the examination of 

 the image may be made by a properly 

 constructed eye-piece, or by the naked 

 eye" it seems to have been overlook- 

 ed that both images should be viewed 



by an eye-piece, or by the eye, not 

 one image by the eye and the other 

 by the eye-piece. In the case given 

 in the paper, fixing the image at a' b' 

 by looking at it from a point back of 

 would make a' b' shorter than c' d\ 

 while its length, determined by an 

 eye-piece, is greater — as the figure 

 shows. Whatever, then, may be the 

 practical difficulty in determining 

 the hitherto accepted angle of aper- 

 ture, it does not appear that anything 

 is to be gained by substituting for it 

 any such angle as the one proposed. 

 F. C. Van Dyck. 



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Uniformity in Oculars. — Among 

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