PHOTOMICROGRAPHY OF DIATOMS 253 



Wash in water, and soak in glacial acetic acid for a day 

 or more, then wash in water again. 



Place on a slide in a saucer of water, and arrange the 

 appendages by means of a needle and brush ; raise the 

 slide from the water, drain it, and make final adjust- 

 ments of the legs, wings, etc. 



Lay another slide down on the first, and tie the two 

 together. Place in methylated spirit for a day or more 

 to harden the object. 



Remove the specimen from between the slides and 

 place in methylated spirit. 



Clear in clove oil. 



Soak in xylene for a few minutes to remove the clove 

 oil, and mount in Canada balsam. 



Photomicrography of Diatoms (Plates 39 to 43). 

 Diatoms present some of the greatest difficulties en- 

 countered in photomicrography. In visual work the 

 greatest care in making all adjustments of the microscope 

 has to be taken in order to display their structure to the 

 best advantage, and this holds good, but to a greater 

 degree, in photomicrography ; for though the detailed 

 structure of many diatoms can be made out with com- 

 parative ease visually, provided the focussing and other 

 adjustments have been carefully carried out, yet it is 

 often difficult to obtain a satisfactory photomicrograph 

 of the object, owing to lack of sufficient contrast. In 

 fact this lack of contrast is one of the chief difficulties 

 to be overcome, particularly with high -power work. 

 Apart from the satisfaction to be obtained from photo- 

 graphing such beautiful objects, the experience gained in 

 making the necessary adjustments is very beneficial to 

 the photographer, and there is scarcely any type of 

 object which offers such good training in accurate 

 focussing, in teaching the value of critical illumination, 

 and in general photomicrographic technique, as diatoms ; 

 and, as the structure of typical forms such as Navicula 



