194 ANIMAL MECHANISM. 



These changes of plane are only found in great movements 

 of the wing. This is an important fact, for it will explain to 

 us the method of their production. Fig. 81 was furnished 

 like fig. 80 by the movements of the wing of a moth (mucro- 

 glossu) ; but on account of its fatigue these movements had 

 lost nearly all their amplitude. 



r r r r- r r r r r- t\;st r r r r ' 



Fio. 81.- -Trncitiff of th" wim; of :i fatigued nvieni'-riossa. The figure 8 i< 

 no longer to be seen, but only ;i siuiplu pcii'lular osci.Uitiou. 



We see only in this figure a series of pcndular oscillations, 

 showing that the wing merely rose and fell without changing 

 its plane. The bright line which borders the ascending and 

 upper parts of these curves is explained by the alternate 

 flexions of the wing as it rubs upon the paper, and shows 

 that the upper surface was rough, and left a distinct trace, 

 while the lower surface presented no simil/ir roughness. 



3. IHn'ctioit of the movement of tlie winy. One more very 

 important element is required to give us a complete knowledge 

 of the movements which the insect's wing executes in its flight. 

 The optical method, while it shows us all the points in the 

 curve described by the gilded extremity of the wing, does not 

 indicate the direction in which this revolution is accomplished ; 

 whatever may be the direction in which the wing moves in its 

 orbit, the luminous image which it it fiords must be always 

 the same. 



A very simple method has furnished a solution of this now 

 (juestion. Let fig. 82 be the luminous image furnished by 

 the movements of tlie right wing of an insect. The direction 

 of these movements which the eye cannot follow is indicated 

 by arrows. 



To determine the direction of these movements, we take a 

 small rod of polished glass and blacken it with the smoke of 

 u wax taper; when holding the rod at right angles to the 

 direction in which the \\ing moves, we present the blackened 



