144 SOME NOTES ON THE METALLURGICAL 



for the purpose. Doubtless other opticians could supply similarly 

 corrected condensers. Condenser A has a longer focus (the actual 

 focal length will depend on the tube length employed and the distance 

 between the condensers), and as it need only be of small diameter, 

 such a high degree of correction as in B does not seem necessary. 



(b) The Vertical Illuminator. — The author does not propose to 

 enter into the question of prism versus disc illuminator to any 

 great extent. The fact that with high powers a disc illuminator 

 must of necessity give and does give far better definition and 

 much more detail, is evident to anyone who has studied the subject 

 or who has critically compared the two illuminators with the 

 same lens on the same field. Examples of this have been pub- 

 lished independently by Rosenhain and by Benedicks; the author 

 (who was unaware of Rosenhain' s work) investigated the matter 

 before Benedicks' paper was published, and his results, though 

 not published, were communicated to some of his friends. It is 

 probably not so well known, however, that the results with low 

 power lenses show the same differences, though not in so marked 

 a manner. The differences produced for any given objective depend 

 on the fineness of detail in the section. Other things being equal, 

 the superiority of the disc becomes more marked as the detail to 

 be reproduced becomes finer. The author always uses a disc 

 illuminator even with the lowest powers, except under exceptional 

 circumstances. (Such may arise in a low power photograph of an 

 object showing no fine detail and possessing very little con1:rast.) 



While, however, the author is convinced of the superiority of 

 the disc illuminator, he has found that many individual discs are 

 very poor specimens, and in this respect he would urge on instrument 

 makers the necessity for more care in choosing material for the 

 " disc." In many cases the glass is so thick and so uneven that 

 the definition of even a low power lens in absolutely ruined. The 

 author a few years ago received an illuminator from one of the 

 largest microscope makers in England — he returned it at once with 

 a note that it was useless owing to the bad glass (giving them details 

 of the behaviour of the disc). The illuminator was returned to 

 him with a fresh glass fitted, which was every bit as bad as the first 

 one. The effect of this bad disc is shown in Figs. 4 and 5. These 

 represent the same field taken with the same objective, ocular, plate, 

 and screen, in fact every condition the same, except that in Fig. 4 

 the disc was a good one, while in Fig. 5 the disc was the bad one 

 mentioned above. It was absolutely impossible to get any sharper 

 definition than that shown in Fig. 5. The author suggests that this 

 is a point to which instrument makers should give far more attention 

 than they do — there is no doubt at all that many of the glasses 

 supplied with disc illuminators are far too thick, and they are often 

 uneven. It is evident also that the discs cannot be very carefully 

 examined by the makers before being put into stock, otherwise 

 such defects would be quickly discovered. 



It may be of interest to mention that the bad disc mentioned 

 above had far more effect on the performance of the 1 in. and \ in. 

 objectives than on the l-6th, probably owing to the larger area 

 of the glass used by the former lenses. Probably this fact and the 



