306 S. C. AKEHURST ON SOME OBSERVATIONS CONCERNING 



denser to be used with high-angled oil-immersion objectives, even 

 though the field for its usefulness may be limited. 



1 hope the photographs illustrating this paper will prove of 

 sufficient interest to stimulate further investigation into the 

 value of annular light, and to demonstrate what limits, if any, 

 should be put upon its use. 



Descriptions of Plates. 

 Plate 20. 



Figs. 1 to 7 are illustrations of various figures of the spectra 

 of Pleurosigma angulatum as seen at the back lens of an oil- 

 immersion objective with the diatom in focus an achromatic 

 condenser, with and without stop, and reflecting condenser being 

 used. Pigs. 1 to 3 are of no special interest just now most of 

 you are familiar with these diffraction spectra, varied according 

 to the diameter of the opening in the iris diaphragm. 



Fig. 1 shows result obtained with the diaphragm almost closed. 



Fig. 2 the diaphragm is opened so that one-third of the back 

 lens is in shadow. The details of the diatom are hardl) T per- 

 ceptible, being flooded out by excess of light. 



Fig. 3. The iris is closed, until two-thirds of back lens is in 

 shadow. In this position, with the spectra just touching the 

 edge of the central beam of light, the best resolution of Pleuro- 

 sigma angulatum is obtained. 



Fig. 4 shows the spectra obtained when a large spot is used. 

 The six diffraction spectra forming the symmetrical image should, 

 however, be slightly moved from the centre outwards to reduce 

 the diameter of the hexagonal spot in the centre, which in the 

 drawing is a little too large. In this instance insufficient light 

 was passed, and an unsatisfactory image of the diatom was dis- 

 played. My next spot being too small, the picture of the spectra 

 obtained is as shown by flg. 5. Here Ave have six dark cuspidate 

 forms, disposed as a six-pointed star, the intermediate spaces 

 being filled with a diffused light, the whole figure being some- 



