150 MEMORANDA. 
movement, at about one inch and two tenths from the centre. 
A small piece of paper, or of gummed label (8), three fourths 
of an inch long and one fourth of an inch wide, attached to the 
upper surface of the slide, where it overlaps the finding-line, 
completes this simple apparatus. It is used thus :—An object 
being in the field to which it is desirous to recur, the observer 
moves his head to the right, so as to get a view of the finding- 
line and label; he then with a pen makes a line on the label 
in apparent continuation of the finding-line. In order to find 
that object at any future time it is only necessary to place 
the slide on the stage, and adjust the microscope so that the 
object may not pass through the field unobserved, then push 
the object-carrier fully down, placing the slide so that the 
line on the label is again in apparent continuation with the 
finding-line, when, by moving the object-carrier upward, the 
object will be seen to enter the field. 
In marking the line on the label it is necessary to observe 
whether the pen is “sighted ” with one or both eyes, and to 
use either habitually, otherwise an inaccuracy in replacing the 
slide will result, which, being magnified by the power of the 
microscope, will prove material. 
It has occurred to me that the finding-line might be 
marked on the object-carrier, or on a small plate attached to 
