ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 



565 



always gives a drawing of the same length. It is necessary in this 

 process to use objectives thoroughly aplanatic. 



In the second process we simply determine the direction of the 

 optic axis in its path from the camera lucida to the table. This direc- 

 tion being known, we shall evidently know the inclination which it is 

 necessary to give to the board so that it may be perpendicular to the 

 optic axis. 



How is the direction of the optic axis to be determinecl ? Being a 

 straight line, it is sufficient to know two points of its path, and this 

 may be obtained in the following manner : — Selecting a very limited 

 point of the field of view and as central as possible, the crossing of 

 two lines of a micrometer in squares, for example, this point is trans- 



FiG. 102 



ferred to the drawing-paper. The latter is placed successively at two 

 different heights — on the table, for instance, and then on a box. 

 When the paper is placed on the table, the point by virtue of the 

 obliquity of the axis naturally falls further from the foot of the Micro- 

 scope than when it is higher, on the box. We ascertain then very 

 exactly (1) the distance between the verticals passing through the two 

 points, and (2) the distance between the horizontals passing through 

 these same points (the height of the box). Then we construct a 

 right-angled triangle having for height the distance between the hori- 

 zontals, and for base the distance between the verticals. The hypo- 

 thenuse of this triangle will evidently give the direction of the optic 

 axis. 



To find that of the inclined plane, it is sufficient to take any given 

 perpendicular to this hypothenuse, or, what is still simpler, to reverse 

 the triangle, making the base the height, the inclination of the 

 hypothenuse being then that which the board ought to have. This 

 may be readily understood, since the angle which the inclined plane 

 makes with the horizontal is evidently equal to that which the optic 



