ZOOLOaY AND BOTANY, MICKOSCOPYj ETC. 



561 



Fig. 100. 



camera lucida of Milne-Edwards and that of Nachet, but so far at 

 any rate as the latter is concerned, the supposed distortion does not 

 exist, at least it cannot be detected by the micrometer. The paper 

 may, however, be of interest from a theoretical point of view. 



" 1. Nature of the Distortions, — These camerae lucidte which are so 

 convenient and so generally employed have the grave inconvenience 

 of giving drawings which do not exactly reproduce the form of the 

 objects drawn, assuming, that is, that the Microscope and the paper 

 are in their usual position — the Microscope vertical and the paper 

 placed on the table or on any other horizontal plane. 



To be assured of this fact draw, for instance, a series of parallel and 

 equidistant lines like the divisions of an object micrometer. Then if 

 the micrometer is placed transversely on the stage of the Microscope, 

 so that the divisions are directed from front to back in the drawing, we 

 shall see that the lines drawn will 

 no longer be equidistant ; they 

 will be the wider apart according 

 as they have been drawn at a 

 greater distance from the foot of 

 the Microscope. If the micro- 

 meter is placed so that the divi- 

 sions are transverse in the draw- 

 ing, the lines will no longer be 

 parallel ; they will diverge from 

 the foot of the Microscope 

 (fig. 100). 



If we draw a square, we shall 

 obtain a trapezium, a quadri- 

 lateral figure of some kind more 

 or less irregular according to the 

 position of the square on the stage 

 of the Microscope, but never a perfect square, A circle will never 

 give a circle, but always an ovoid, &c. 



These distortions are little noticeable so long as the parts drawn 

 occupy only a small portion of the field of view, and so they may be 

 disregarded if absolute exactness is not required. But if the micro- 

 scopic field is large, the distortions are considerable. A square, for 

 instance, has given me a trapezium, the small base of which was 

 114 mm., and the sides 136. There were therefore differences in the 

 drawings of equal lines (the sides of a square) amounting to 22 in 

 114, exceeding 19 per cent. 



We see therefore to what errors we may be exposed if we use such, 

 drawings for exact measurements— measurements of the histological 

 elements, of magnifying power, &c. 



2. Cause of these Distortions. — The distortions are due to the fact 

 that in these camerse lucidse the two surfaces at which the total reflec- 

 tions take place are very close to one another, so that the reflection 

 upon the table is not made along a vertical axis. In fact, if it were 

 so, a part of the image would be seen on the stage and foot of the 

 Microscope, and could not be received on the paper : such an obliquity 



Ser, 2.— Vol. III. 2 



