THE DESCRIPTION OF A LEPIDOPTEROUS OVUM. 197 



show. No one will probably try to describe an egg that does not 

 already know something about them, but the most expert may omit 

 some important detail, if without a scheme, a memoria teclmica. 



A description of an egg under a hand lens only is practi- 

 cally of little or no use, though it may sometimes be better than 

 nothing." 



While fully recognising the logic of the above remarks on most of 

 the questions raised, I do not consider that any points should be 

 omitted from the schedule, but rather that they should be included 

 with some indication that such and such points and details are far 

 better registered by photography. Their inclusion serves to direct 

 the attention of the observer to the particular aspects of the 

 points, which are necessarily to be kept in view to obtain the desired 

 result. 



Accurate measurements are of the utmost importance. Roughly, 

 one can obtain the relative sizes of eggs by reference to the photo- 

 graphs if, and the if is emphasised, they are all done to a standard 

 scale. On this subject Dr. Chapman says {in litt.) : — " The results of 

 even the best photographers vary fractionally from the scale of 

 enlargement they give, and to secure accuracy, the best method I find 

 to be to make a camera outline of the egg from each point of view, 

 and, at the same time, and on the same paper, mark under the camera 

 from a micrometer scale. Such outlines may sometimes serve instead 

 of those photographs that are only given for the sake of form and 

 measurement." 



Mr. F. Noad Clark has very kindly sent me on the following 

 details of two methods of taking the measurement of ova under the 

 microscope. He says : — " One method is by means of a stage 

 micrometer. This is a glass slide ruled in yij^ths and y^^j^yths of an 

 inch or -^-^ih?, and x^oths of a millimetre, and costing five shillings. 

 The object is focussed through the microscope with camera attached, 

 as in photomicrography, and the image projected on the ground glass 

 screen is carefully measured with a pair of compasses. The ovum is 

 then replaced by the stage micrometer, and the corresponding 

 measurement of the image of its divisions taken." 



Another method, and the one generally adopted, is by using, in 

 addition to the stage micrometer, an eyepiece micrometer. This is a 

 glass disc ruled with arbitrary divisions, and is placed upon the 

 diaphragm of the eyepiece; the number of divisions corresponding with 

 the object is then noted and read off on the stage micrometer, which 

 has meanwhile replaced the object or egg. This method entails the 

 purchase of an eyepiece micrometer, costing five shillings. The 

 "Ramsden" eyepiece micrometer is probably the most convenient of 

 all methods. It is, however, somewhat costly. 



For the measurement of the depth or thickness of an object, some 

 microscopes have the fine adjustment screw so graduated that each 

 turn of the screw represents a proportionate measurement in millimetres. 



It follows, of course, that, given a photograph or drawing of an egg 

 magnified by a certain stated number of diameters, it is easy to 

 estimate the size of the original by dividing the magnified or reproduced 

 image by the number of diameters." 



* This is my method, F.N.C. 



