VARIOUS MODES OF ILLUMINATION LARGE CONES 423 



with no idea of offering opposing opinions to those of Professor Abbe ; 

 we have no direct judgment, but we record these facts as factors in 

 and for the elucidation of the question. It is perhaps better to test 

 the j on some of the more minute hairs which are studded over the 

 delicate lining membrane. The same results will be obtained. Thus 

 it would appear to suggest itself that this glaziness depends on the 

 relation of the aperture of the illuminating cone to that of the objective 

 cone. Apochromatic objectives behave precisely as achromatic ob- 

 jectives in this respect. Of course, if the hair becomes pale and in- 

 distinct on the opening of the iris, it shows that there is uncorrected 

 spherical aberration in the objective ; another objective must there- 

 fore be used ; that paleness has nothing whatever to do with the 

 glaze or false light mentioned aboye. 



In photo-micrographs of bacteria one frequently sees a white halo 

 round them. We have never been able to demonstrate what this is ; 

 sometimes it denotes the presence of an envelope, and sometimes it 

 is the result of the use of too small a cone of illumination. Photo- 

 micrography with a small cone is quite easy, as great contrast can 

 be secured. With a large cone the difficulties begin difficulties 

 of adjustment, difficulties of lens correction, difficulties of exposure, 

 and difficulties of development. If, so far as our experience goes, 

 a good photo -micrograph is required, these difficulties must be 

 mastered. 



It is hardly necessary to remind the student that in micrometry 

 it is essential that the edges of the object should be defined ; conse- 

 quently a large cone must then be employed. 



For the examination of Poly cystines, Foraminifera, &c., a binocular 

 is useful ; illumination may be by a Lieberkiihn if mounted dry, and 

 by dark ground by a condenser if mounted in balsam. Parts of 

 insects should be usually examined with dark-ground illumination ; 

 whole insects are seen best with the Lieberkiihn, and the binocular 

 should be used for both. 



Some of this class of objects are best seen under doitble illumina- 

 tion ; that is, a dark ground with a condenser and light thrown from 

 above with a silver side-reflector, as the Lieberkiihn cannot be used in 

 conjunction with an achromatic condenser. It is a good plan with 

 low -power Lieberkiihn work to interpose between the slip and the 

 ledge a strip of plain glass Vinch wide ; this prevents the ledge 

 stopping out light from the Lieberkuhn when it is larger in diameter 

 than the slip. Mr. Julius Rheinberg has recently brought to a 

 high state of perfection a system of colour illumination, and the 

 special importance of the choice of suitable colours. It is of much 

 interest, but cannot be condensed in the space at our disposal. The 

 full paper will be found illustrated in Journ. E.M.S.' 1896, p. 373, 

 and the 'Journ. R. M. S.' for 1899, p. 142. 



Polarised light used with a condenser is very useful for insect 

 work. For very low-power work such as the usual botanical sec- 

 tions it is a good plan to give up the cone, and place a piece of fine 

 ground glass at the back of the condenser ; and with lamplight it is 

 as well to use a Clifford's screen with it. With objectives of greater 

 angle than '6 N.A. it is usually difficult to get satisfactory illumina- 



