14 



INTRODUCTION. 



microscleres is then scraped off with a knife and immersed in a small drop of 

 water placed on the quartz-slide. In this water they disperse and when they 

 are scattered a little gum may be added and the whole allowed to dry. The 

 spicules then appear attached to the quartz-slide. A small drop of the chloral 

 hydrate glycerine is put on and this covered with a quartz-cover. 



The slide thus prepared is placed on the stage of the microscope, and a 

 spicule to be photographed sought with ordinary light and ordinary lenses. 

 When a spicule of which a photograph is desired is found, the ordinary lenses 

 are replaced by a quartz-objective and a quartz-eyepiece, the spicule brought 

 to focus and centred, the ordinary light switched off and the magnesium-u. 

 v. light switched on. A disc of uranium-glass with a lens attached at a slightly 

 oblique angle is then placed over the eyepiece and the tube of the microscope 

 lowered until a sharp fluorescence-image of the spicule becomes visible on the 

 uranium- glass. When the spicule is thus focused for the 280 /i/<-light the 

 uranium-glass arrangement is removed and the camera placed over the micro- 

 scope. 



As it is difficult to focus quite correctly with this fluorescent arrangement 

 and as in many cases the shape of the microscleres can be clearly revealed only 

 by a series of photographs obtained by focusing at different levels, I usually 

 took four differently focused photographs of each spicule. After having focused 

 the spicule and put the camera on, I usually raised the tube of the microscope 

 with the micrometer-screw 3 At this level I took the first photograph. 

 I then lowered the tube 2 [i and took a second photograph, and so on, the third 

 4 /< and the fourth 6 /< lower than the first. The intervals between these levels 

 were sometimes more, sometimes less. Four such photographs 6 by 9 cm. can 

 easily be taken rapidly in succession by means of a sliding-plate arrangement. 



I found the most useful combination of lenses to be immersion-quartz 

 monochromat 1.7 mm., quartz-eyepiece 10. This gives, with a suitable length 

 of camera, a magnification of 1800. Most of the u. v. photographs of the micro- 

 scleres on the plates accompanying this report have been taken with this com- 

 bination. 



I have taken some u. v. photographs without an eyepiece. In these cases 

 the focusing was done by means of a uranium-glass with attached lens, placed 

 in a ruler-like frame, which was laid across the (open) top of the camera. In 

 this way excellently defined photographs can be procured but, the magnification 

 even with the 1.7 mm. quartz monochromat and the camera drawn out long, 

 is insufficient. 



