28 DRAWING AND DETERMINING THE SPECIES OF DIATOMS. 



Accessory Apparatus. There is little that need be said about accessory 

 apparatus, as it is such as is generally employed by microscopists. A 

 heliostat is sometimes useful for observation in monochromatic light and 

 for photo-micrography. Reference will hereafter be made to the camera 

 lucida and also to microscopical preparations. 



3. Drawing and Determining the Species of Diatoms. 



A careful determination of any diatom is impossible without previously 

 making a good drawing of it. It is only by comparing this drawing with 

 those published by various authors that any given form can be studied. 

 The diatomist therefore ought to have at all times the camera lucida 

 ready at his side. I have tried every apparatus of this kind, but for 

 working with ease I know of none which can compare with the camera 

 lucida for an inclined microscope as constructed some years ago by 

 Mr. A. Nachet ('). This apparatus resolves the most delicate details 

 without any part becoming deformed, but the plane of the paper on 

 which the drawing is made should be parallel to the surface of the 

 prism to which the eye is applied. In drawing diatoms, hard pencils 

 (such as Gilbert's No. 5) should be used, and they should be very 

 finely pointed, otherwise it would be quite impossible to represent very 

 close striae sufficiently near to one another without confusion. 



It is essential that all drawings should be of the same magnification, 

 which should be sufficiently large to reproduce delicate details. All the 

 drawings in the Atlas to my Synopsis des Diatomees de Bel^ique were 

 drawn to 900 diameters and reduced by photography to 600. This 

 magnification was not chosen at random. I fixed upon it in order that 

 my drawings should bear comparison with those of previous good 

 authors, such as William Smith, Greville, Gregory, &c, whose figures 

 were made at 400 diameters ; mine are therefore half as large again as 

 the figures of W. Smith and the other authors previously mentioned. 



It is to be regretted that Dr. Adolph Schmidt has used so arbitrary 

 a magnification as 660 for his Diatom Atlas. 



In the present state of the science a magnification of 400 diameters is 

 no longer sufficient for an original drawing, and as far as possible all 

 drawings should be 900 diameters, while certain details even require 

 double that amplification. 



In the present work the drawings of Genera have been made at 900 

 diameters, and wherever possible have been reduced to 600. With regard 



(') The Microscope: English Edition, London, 1893, pp. 92-93. 



