1889*] MICROSCOPICAL JOURNAL. 251 



of any two microscopes, and' that it is a factor to which others have no 

 reference. Moreover, all who wear spectacles for " long sight v know 

 that the focus, and, therefore, the distance between the eye-glass and 

 the stage, depends slightly on whether the observer wears or dispenses 

 with his spectacles. And I would remind those who attach special im- 

 portance to the diameter of the field that this feature is dependent not 

 on tube length, nor on the power of the objective, nor of the combina- 

 tion of eye-piece and objective, but simply on one particular lens in the 

 ocular, viz., on the field glass, so called because it determines the dimen- 

 sions of the field. As to the apparent size of the object, I shall show 

 directly on what this depends ; that it is a constant for a given combi- 

 nation (of eye-piece and objective) and a given tube-length ; and that 

 the apparent size itself bears no relation to the distance of projection, 

 i, e., to the space between eye-glass and paper. 



The desirability of having a fixed rather than a varying distance as a 

 standard is obvious : though the magnification of the sketch of object may 

 be increased in proportion to its distance from the eye-piece, the mag- 

 nifying power of the microscope will not vai'y. The apparent size of 

 the image depends not on the nearness or remoteness of the paper ; but, 

 as in ordinary vision, it has to be measured by the angle which the 

 thing seen, or image, subtends at the eye, and this angle remains con- 

 stant whatever may be the distance of the paper. To give a concrete 

 example. Using an Economic ^-inch on my " Star " stand, the draw- 

 tube closed, and the A eye-piece at three feet from the table, the T i- F 

 inch is projected by Beale's reflector to an extent which measures so 

 nearly 3'' that we may fix it at 3" ; at 2 feet it occupies a space of 2" ; 

 at 1 foot of 1 ; at 10" of a trifle over ^ ; at 6" of half an inch ; at 3" of 

 one-quarter of an inch. You will already see a law underlying these 

 measurements with which we need not trouble ourselves in a practical 

 paper ; though I shall show you directly how to apply it for enlarging 

 and reducing drawings. At the surface of the reflector itself, the pro- 

 jected image of T oV — indeed every object viewed — as may be shown by 

 diagram, almost occupies the immeasurable space of a mere vanishing 

 point, be the power of the objective what it may. During all these 

 variations of distance the apparent size of the magnified yvV remains 

 the same. Why? Because the angle it subtends at the eye does not 

 vary, though you may vai'y the distance of projection. It is obvious 

 from this that it is necessary to fix on a standard distance, which will 

 be of practical value to observers all the world over ; it is equally ob- 

 vious that the varying standards we have discussed in the preceding 

 paragraph may safely be rejected. The question is, what standard 

 should be substituted for them ? Ten inches has been adopted in pref- 

 erence to any more arbitrary distance, by analogy to the average focal 

 length of the human eye, which is ten inches. This standard of distance 

 is preferable in practical microscopy to any variable standard dependent 

 on the sight of the individual observer ; or the distance at a given time 

 of the eye-glass of a given ocular, from the stage of a given micro- 

 scope ; or the field-determining power of a given field-glass. 



If, however, you desire for any i-eason to use varying standards of 

 height, then you ought in every case to draw a micrometric scale along- 

 side of each object you sketch, projecting that scale carefully for each 



