PROF. ZETTNOW ON A. PELLUCIDA. 287 



show up vividly destroy any kind of picture, must be entirely 

 eliminated, or show in a slight, non-interfering degree only. 



After spectroscopic trial of a large number of violet fluids, 

 the best filter proved to be a solution of iodine in chloroform ; 

 this of a suitable concentration only permits the transmission 

 of red and violet rays, so that by absorption of the former by a 

 cupric-ammonia filter one can make use of violet light only. 



If the solution contains a half per cent, of iodine, a layer 

 6 mm. thick allows of working with rays from G to H alone, 

 directly the red rays are absorbed by a cupric-ammonia filter 

 of such a concentration as to pass rays from F onwards. 



For this copper-iodine filter, therefore, two cuvettes (con- 

 tainers) are required, as, firstly, the active substances are not 

 chemically indifferent as in the copper-chromate filter, and, 

 secondly, the fluids will not mix together. 



As the eye is but slightly sensitive to violet rays, the field 

 of vision is much darkened ; even with bright sunshine this 

 loss of light is disagreeably felt. Realgar mounts are inad- 

 missible, as the yellow colour of this substance completely ab- 

 sorbs the violet rays. In taking the Amphipleura reproduced in 

 phototype in Fig. 858 x 6,400,* therefore, I used a preparation 

 by J. D. Moller mounted in iodide of mercury; the illumina- 

 tion was as much as possible oblique, and the aperture so small 

 that strongly marked diffraction lines made their appearance. 

 The impression was taken on a slow erythrosin plate covered 

 with No. 140 of my emulsion, and for strengthening the 

 contrast the negative was first treated with silver and gallic 

 acid, and then intensified with mercury and sulphate of 

 ammonia ; it was only in this wise possible to properly repro- 

 duce on the plate the complete resolution of the diatom as seen 

 by the eye by the contrast of light and shade. Avoidance of 

 the diffraction by using a larger aperture was impossible, as 

 the delineation became too pale for photographic impression 

 (apochromatic N.A. 1'4 Zeiss). 



The proximate conditions under which the negatives for 

 Figs. 779, 788,* and 791 were produced are as follows : — In all 

 an aperture of i-^=0'3-0'25 of the apochromatic was em- 

 ployed, the illumination as oblique as possible ; the cupric- 

 ammonia filter tested by sunlight passed rays from about 



* The figures refer to the original paper published in Eder's " Jahrbuch 

 f. Photogr. u Reproductionstechnik,* f. 1893. 



