12 



INTRODUCTION. 



does not always pay sufficient attention to those other points which seem unim- 

 portant. This will often lead to the omission or inadequate representation in 

 drawings of such characters as seem unimportant, whereas, in the mechanically 

 reproduced photographs, they will be just as fully and correctly shown as all 

 other points. It is quite probable, that at some future time, when science 

 is more advanced, one or more of the points considered unimportant now, and 

 consequently not carefully represented in drawings, may become important. 



These considerations induced me to represent the sponges and their parts 

 in my reports on the Tetraxonia of the Deutsche Tiefsee-Expedition (Lendenfeld, 

 loc. cit.), and the Deutsche Siidpolar-Expedition ' so far as possible photo- 

 graphically. During the years I was engaged in preparing these reports I 

 gained considerable experience in this photographic work. By the construction 

 of a diaphragm, placed just above the objective, I overcame the difficulties 

 which had formerly prevented a photographic reproduction of the megascleres; 

 and though I was hampered by the quality of my microphotographic outfit 

 and the insufficiency of the light at my disposal I obtained quite satisfactory 

 photographs of these spicules. My attempts similarly to dehneate the micro- 

 scleres were not nearly so successful and I was frequently obliged to draw them. 



When I began the study of the Sponges collected by the "Albatross" I ob- 

 tained in the first place a first-class microphotographic outfit, replaced the 

 Welsbach burner, formerly used as a source of light, by a Nernst lamp with 

 crossed rods and was thus enabled to improve considerably the microphoto- 

 graphic reproductions of the sections and megascleres. The difficulty of photo- 

 graphing the microscleres with high powers, however, still remained. Like the 

 megascleres the microscleres consist of colourless, transparent silica and are 

 rendered visible only by the difference in the refractive indices of the silica and 

 the surrounding medium. When, as is the case in the spines and other parts 

 of most microscleres, such colourless structures are very thin, less than a light 

 wave-length in diameter, no sharply defined image of them can be produced 

 on the photographic plate, no matter how excellent the lenses of the micro- 

 scope may be. The only way to get well-defined images of them is to make use 

 of light of shorter wave-length; the lower limit of the size (thickness) of 

 minute structures of this kind, still clearly reproducible microphotographically, 

 must, ceteris paribus, be in inverse proportion to the wave-length of the light 

 employed. The shortest light-waves obtainable would accordingly be the best 

 for work of this kind. Since, however, the employment of very short hght-waves 



' R. V. Lendenfeld. Tetraxonia. Deutsche Sudpolar-Expedition, 1901-1903, 1907, 9. 



