MEASUREMENT OF ORGANISMS. 3 



graphic paper. More or less transparent organs, such as 

 leaves, petals, insect-wings, and appendages of the smaller 

 Crustacea, may be reproduced either directly on blue-print 

 paper or by "solar prints," either of natural size or greatly 

 enlarged. For solar printing the objects should be mounted 

 in series on glass plates. They may be fixed on the plate by 

 means of balsam or albumen and mounted between plates either 

 dry or in Canada balsam or other permanent mounting media. 

 Wings of flies, orthoptera, neuroptera, etc., may be prepared 

 for study in this way; twenty-five to one hundred sets of wings 

 being photographed on one sheet of paper, say 16 X 20 inches 

 in size. Microphotographs will sometimes be found service- 

 able in studying small organisms or organs, such as shells of 

 Protozoa or cytological details. 



Camera drawings are a convenient although slow method of 

 reproducing on paper greatly enlarged outlines of microscopic 

 characters, such as the form and markings of worms and 

 lower Crustacea, sponge spicules, bristles, scales and scutes, 

 plant-hairs, cells and other microscopic objects. In making 

 such camera drawings a low-power objective, such as ZeissA*, 

 will often be found very useful. 



The Determination of Integral Variates. 

 Methods of Counting. 



While the counting of small numbers offers no special diffi- 

 culty, the counting becomes more difficult with an increase of 

 numbers. To count large numbers the general rule is to di- 

 vide the field occupied by the numerous organs into many 

 small fields each containing only a few organs. Counting 

 under the microscope, e.g., the number of spines, scales or 

 plant-hairs per square millimetre, may be aided by cross-hair 

 rectangles in the eyepiece. The number of blood-corpuscles 

 in a drop of blood, or of organisms in a cubic centimetre of 

 water, have long been counted on glass slides ruled in small 

 squares. 



electric lamps such as are fed by a single cell give sharp shadows of 

 small objects. 



