INTRODUCTION. 7 



inanimate object which was made to slide along a straight 

 edge and dots were repeatedly made on a glass-plate; when 

 these were joined, the result ought to have been a perfectly 

 straight line, and the line was very nearly straight. It may be 

 added that when the dot on the card was placed half-an-inch 

 below or behind the bead of sealing-wax, and when the glass- 

 plato (supposing it to have been properly curved) stood at a 

 distance of 7 inches in front (a common distance), then the 

 tracing represented the movement of the bead magnified 15 

 times. 



Whenever a great increase of the movement was not required, 

 another, and in some respects better, method of observation was 

 followed. This consisted in fixing two minute triangles of thin 

 paper, about ^ inch in height, to the two ends of the attached 

 glass filament ; and when their tips wore brought into a line so 

 that they covered one another, dots were made as before on the 

 glass-plate. If we suppose the glass-plate to stand at a dis- 

 tance of seven inches from the end of the shoot bearing the 

 filament, the dots when joined, will give nearly the same figure 

 as if a filament seven inches long, dipped in ink, had been 

 fixed to the moving shoot, and had inscribed its own course 

 on the plate. The movement is thus considerably magnified; 

 for instance, if a shoot one inch in length were bending, and 

 the glass-plate stood at the distance of seven inches, the move- 

 ment would be magnified eight times. It would, however, have 

 been very difficult to have ascertained in each case how great 

 a length of the shoot was bending; and this is indispensable 

 for ascertaining the degree to which the movement is magnified. 



After dots had been made on the glass-plates by either of 

 the above methods, they were copied on tracing paper and 

 joined by ruled lines, with arrows showing the direction of the 

 movement. The nocturnal courses are represented by straight 

 broken lines. The first dot is always made larger than the 

 others, so as to catch the eye, as may be seen in the diagrams. 

 The figures on the glass-plates were often drawn on too large 

 a scale to be reproduced on the pages of this volume, and the 

 proportion in which they have been reduced is always given.* 

 Whenever it could be approximately told how much the move- 

 ment had been magnified, this is stated. We have perhaps 



• We are inucli indebted to lie has reduced and engraved our 

 Mr. Cooper for the care with wliicli diagrams. 



