SUB-STAGE ILLUMINATION. 307 



what ill-defined. This effect was due to an excess of light ; by 

 slightly closing the iris diaphragm the light was reduced, and we 

 have the result as shown in fig. 6 the symmetrical design well 

 defined on a black ground, and just a glimpse of another portion 

 of the spectra at six points round the shadow caused by the partly 

 closed diaphragm. With the spectra showing, as illustrated in 

 fig. 6, I obtained the best definition of Phurosiyma angulatum 

 with achromatic condenser and spot. 



Fig. 7 is the record of the spectra obtained of the same diatom, 

 using the reflecting condenser ; the similarity between the figures 

 7 and 5 is noticeable. 



My reflecting condenser to work at its best when using it for 

 annular light requires the light cut down until a crisp image is 

 shown of the spectra as at fig. 6. 



Fig. 8 represents the rulings on an Abbe test plate, as displayed 

 by T V inch oil-immersion objective and reflecting condenser. The 

 position of the light bars is to be noted : there are six those at 

 the top and bottom are not quite fully displayed. On first 

 examining the back of the objective I observed the two rows 

 of six white dots, as shown at fig. 9. At another examination 

 the light probably being more central I found an almost com- 

 plete circle, as at fig. 10, made up of ten white clots on each side 

 and a thin streak of light at the bottom. I have not yet been 

 able to put forward a suggestion as to how these are formed. 

 I have, however, included them in my record, as they may be 

 of some interest. 



Plate 21. 



Fig. 1. Nitzschia linearis x 2,500, showing the white-dot image. 

 This photograph was taken with a highly corrected oil-immersion 

 condenser and axial illumination. 



Fig. 2. Nitzschia linearis x 3,000, this time showing the 

 black-dot image. This photograph was taken with reflecting 

 concentric condenser. 



Journ. Q. M. C, Series II. No. 74. 22 



