354 SUMMARY OF CUREENT RESEARCHES RELATING TO 



beam passing through the objective which destroys the sharpness of the 

 image, and it is the small W.A. (i.e. too large an unutilized area in the 

 objective) which coats the image with black-and-white diffraction images. 

 This image in the Ramsden disk (tig. 71, No. 1) should be compared 

 with No. 2, which shows that the concave mirror is in correct adjust- 

 ment, therefore the flat of the flame is imaged properly and centred to- 

 the disk. 



No. 3 illustrates a properly set-up illumination with the plane 

 mirror. The only difference is that the image of the flat of the flame 

 is smaller, and the unutilized portion of the objective larger ; which is, 

 as we have just seen, disadvantageous. With daylight illumination we 

 must substitute image of window for image of lamp-flame. This image 

 will vary according to circumstances. It may be, for instance, a gap 

 between chimney-pots or perhaps between houses. It is remarkable that 

 these fundamental principles of elementary microscopical manipulation 

 have never been explained in any text-book on the subject. 



The Ramsden disk is an image of the back lens of the objective. If 



900000 



Fig. 71. 



the Ramsden disk be too small for examination by the unaided eye, the 

 eye-piece may be removed and the back lens itself be examined ; but 

 probably the simplest and qnickesc method is to employ a " loup," for it 

 saves the trouble of removing and replacing the eye-piece. 



Ground (Jkiss. — I became first acquainted with ground glass in 1875, by 

 purchasing a Swift's excellent Universal Condenser, which he had brought 

 out the previous year. This condenser (an improvement on Hall's, made 

 by Swift in 1<S68) I still have, and use.* The top lens is removable; the 

 back, consisting of two doublets, form the best possil)le condenser for 

 low-power work ; a blue-glass light-modifier for lamplight fits below these 

 lenses, and a ground glass (which is never used) fits above them in place 



* So far as I am aware, these condensers were the first for low powers ever 

 constructed, and the microscopical world is greatly indebted to the late James 

 Swift, not only for them, but also for many excellent improvements, both in the 

 brass and glass of the Microscope. 



This particular condenser is, I understand, no longer made ; therefore a fuller 

 description, showing wherein it differs from its modern substitute, is necessary. 

 This condenser has an uncorrected front lens, and a pair of achromatized doublets 

 at the back ; therefore, when used as a whole, it is under-corrected, but neverthe- 

 less it makes a useful condenser for ordinary work with the medium powers (say, 

 J and ^). When, however, the top is removed, a perfectly achromatic combination 

 is obtained, which is, as I have already stated, the best ever constructed. Its 

 modern substitute, for which that particularly fine combination of Baker's may 

 be taken as a type, has also an uncorrected front lens ; but the backs are over- 

 corrected, and so the condenser as a whole is perfectly corrected. Now, when the 

 top of this modern condenser is removed you do not find such a perfect low-power 

 condenser as with the old form, because the combination is now over-corrected. 



