22 PRACTICAL BOTANY 



off. With regard to this method of drawing the following 

 remarks are to be noted : 



i. The adjustment of light is sometimes difficult, so that the 

 object and the point of the pencil may be seen simultaneously 

 with equal clearness : if the illumination of the field be too faint, 

 the paper is to be darkened by a screen placed between it and the 

 window, while if the field be too bright the tinted glass which is 

 usually supplied with the camera may be adjusted so as to cut off 

 part of the light which comes through the instrument. 



ii. This method of drawing, though convenient, is open to 

 objection on the ground of distortion of the image : to avoid this, 

 an inclined desk may be used, so adjusted that the plane of the 

 paper may be perpendicular to a line drawn from the camera to 

 the centre of the image projected on the paper. But where the 

 area of the object is not great the distortion may be neglected, 

 provided the drawing lie made as near as possible to the foot of the 

 microscope. 



iii. In drawing with the camera it will be found convenient 

 first to take a trace in very faint lines, and then, removing the 

 camera, to finish the drawing, and put in the details with a free 

 hand, always maintaining a close observation of the object. 



iv. An exact measure of the scale of the drawing under any 

 given power may be obtained by the following simple method. 

 Take a stage micrometer with parallel lines ruled on it at distances 

 of j-^ooths of an inch : draw these lines by help of the camera 

 under the magnifying power which it is desired to measr.re, and 

 then with an ordinary rule ascertain the distance between the 

 lines drawn : divide this into ^^ths of an inch, and the result 

 is the scale of enlargement of the drawing. 



v. On every drawing made with the camera the magnify ina 

 power should be noted at once, as well as the number or letter of 

 the eye-piece and objective. 



Measurement of Objects. Measurements may be 

 most readily made by means of an eye-piece micro- 

 meter, which is a glass slip, fitted into the eye-piece, 

 and having a scale engraved upon it. The value of the 



