INTRODUCTION. 13 



for microphotography is connected with great technical cUfficuIties, we must 

 for the present content ourselves with light-waves 275-280 /<,« long, that is 

 about half of the length of the waves of ordinary light. Even this light is not 

 convenient to work with, because it is quite invisible to the human eye and does 

 not jxiss through glass. All the optical parts of the apparatus used for its 

 employment must be made of some material like quartz, and fluorescent sub- 

 stances like uranium-glass must be utilized for making the image obtained 

 visible and focusable. An apparatus for microphotography with light of these 

 wave-lengths has recently been made by Koehler (Zeiss) of Jena. 



To overcome the difficulties previously experienced in photographing the 

 microscleres I obtained this apparatus and succeeded, after a number of trials, 

 in producing satisfactory photographs also of these spicules. 



To work this apparatus 4.8-5 amperes of the ordinary three phase-current 

 of 120-125 volts supplied by the Prag Municipal Works were employed. This 

 current is converted, in a transformer, to one of from 15,000 to 15,560 volts, 

 which is made to pass, in sparks, between cadmium- or magnesium-electrodes. 

 The sparks are strengthened by eight Leyden jars attached to the circuit, and 

 are very brilliant. For photographing the microscleres of sponges I soon found 

 magnesium-electrodes more suitable than cadmium-electrodes, and I have 

 worked with the former ever since. The magnesium spark-light produced 

 passes through a quartz-lens and two quartz-prisms. In the ultraviolet part of 

 the magnesium-spectrum thus obtained there is an exceedingly intense line 

 produced by light of a wave-length of 280 /</(. This light was used. It passes 

 through another quartz-lens, is reflected upwards by a quartz-prism placed 

 below the microscope, and concentrated and thrown on the preparation by a 

 quartz-condenser. 



After many trials I found the following the best way to make the micro- 

 sclere-preparations to be photographed with this light: to make a sterraster- 

 preparation, a quartz-slide is covered by a thin layer of gum and, before the 

 gum is dry, a number of these microscleres, previously obtained by sedimenta- 

 tion, are allowed to fall on the gummed quartz-slide. This is then placed in 

 the thermostat oven. When it is quite dry, a small drop of a concentrated 

 solution of chloral hydrate in glycerine, a liquid with sufficiently high refractive 

 index and permeable to these ultraviolet rays, is put on and the whole covered 

 with a quartz-cover. To make a preparation of the euasters the minute spic- 

 ules obtained by centrifuging are spread out on an object glass and dried, 

 whereupon they appear quite firmly attached to the glass. A portion of these 



