1882.] 



MICKOSCOPICAL JOUENAL. 



67 



stancy of the angle at the principal 

 focus of the instrument used as a mi- 

 croscope. The significance of this 

 angle, and its importance, is therefore 

 partly found in the fact that it is the 

 angle within which the actual field of 

 view enlarges as the tube of the in- 

 strument is shortened, and working- 

 distance is increased. 



The differences between the angles 

 of the telescope and that of the mi- 

 croscope are seen in the above table. 

 The approximate coincidence in the 

 two, in the case of the ^-inch object- 

 ive when used with the long tube, shows 

 that the different use may have dif- 

 ferent conditions, producing special 

 variations. In that case, the so-called 

 " available " front was so much less 

 with the instrument used as a micro- 

 scope, that the angle at the nearer 

 focus was nearly the same as that at 

 the further one. 



It is, of course, apparent that all 

 the above tables and measurements 

 could be made much more exact by 

 computation, if we were in possession 

 of the full formulae for the construc- 

 tion of the objectives in question, but 

 in the absence of such knowledge, 

 the experiments seem sufficient to il- 

 lustrate the principles. 



These observations suggested an- 

 other series which is their natural 

 complement, viz.: the investigation of 

 the practical effect of stops placed 

 upon the front of the objective. For 

 this purpose, circular discs of black 

 paper were used, beginning with those 

 of the size of the so-called " available" 

 field in the cases before described, 

 and enlarging them till the whole 

 field of view of the microscope was 

 stopped out. This was only done 

 when using the instrument micro- 

 scopically, for the results would be 

 similar in kind in either case, and my 

 purpose was to see how much of the 

 front in the glasses named is useful 

 by contributing image-making rays 

 which go to the completion of the 

 definition, brilliancy, and perfection 

 of the total image examined in the 

 ocular. 



In the case of the three-inch ob- 

 jective, I began with a stop .3-inch in 

 diameter. With the long tube, the 

 only difference noticed was a slight 

 lack of good definition in the centre 

 of the field. Second, a stop of .35- 

 inch was used. A small round shad- 

 ow was now visible in the centre of 

 the field, almost a black dot in its 

 centre, and shading gradually away. 

 Its effect as shadow was seen on about 

 one-sixth the diameter of the field, 

 rather less than more, and the divi- 

 sions of an ivory scale could be read 

 through it, quite to the centre of the 

 field. Third: A stop .42-inch was 

 used. The diameter of the lens clear 

 of the brass mounting, was carefully 

 measured and found to be .46, so that 

 the annular rim of the lens only .02- 

 inch wide was in use. There was now 

 a black spot in the centre of the field, 

 stopping out rather less than one- 

 quarter in diameter of the whole, 

 shading away with a penumbrous 

 margin. The fibres of paper, or the di- 

 visions of the ivory scale were plainly 

 visible except in the central quarter 

 of the field, but the definition was a 

 good deal impaired. The scale show- 

 ed that the field was undiminished in 

 diameter, and that the black dot oc- 

 cupied ^V of the whole of it. 



I now began shortening the tube, 

 and a bluish shadow began to creep 

 into the field from the margin, whilst 

 the black dot in the centre of the 

 field grew smaller. When the draw- 

 tube was quite closed the central 

 black spot was reduced to a mere 

 point, evidently the apex of a conical 

 shadow touching the centre of the 

 field-glass of the ocular. The shad- 

 ows at the margin had become denser 

 until they stopped out all but the 

 central third of the field, when the 

 object was still visible. 



Precisely similar phenomena were 

 seen in experimenting with the other 

 lenses. The' clear front of the i^- 

 inch measured .45-inch. A stop .39- 

 inch in diameter left a very narrow 

 annulus at the margin of the field, in 

 which the scale divisions were seen. 



