1876.] 
Correspondence. 
133 
was absent from Paris when your 
reclamation was produced on the 
subject of the figure described in 
space by the wing of inse&s and 
birds ; it is on that account that I 
have delayed to answer your note. 
It appeared to me that it was best to 
address myself to you diredtly. 
Indeed, I believe there exists between 
us a pretty considerable disagreement 
as to the manner in which the figure- 
of-8 described by the insedt is gene- 
rated. I do not hesitate to acknow- 
ledge that in your memoir you have 
pointed out certainly before me the 
figure-of-8 presented by the wing of 
the insedt during flight, and I regret 
not having had earlier knowledge of 
your work in order to assign to you 
that priority. One of my audience, 
however, told me last year that 
Lacordaire had formerly pointed out 
indicate. In the copy of my work 
which I have the honour to address 
to you you will see at page 171, 
figure 16, that I find that the two 
branches of the 8 are engendered by 
two movements alternating and in 
opposite directions produced by the 
wing of the animal, as is shown by 
the reverse diredtion of the two 
arrows. In my experiments I applied 
gold leaf to a point of the extremity 
of the wing, and it is the track made 
by this single point which describes 
the contour of the 8. In your diagram 
you ascribe to the two opposite 
borders the formation of the figure-of- 
8, so that (diagram 5) the thin border 
which is formed above at the be- 
ginning of the movement passes 
below at the end of the movement. 
For me the narrow border does not 
pass thus from one side to the other 
that figure of movement of the wing ; 
but I have not succeeded in finding 
that indication in his works on 
entomology. In regard to the inter- 
pretation of that appearance of the 
wing in motion, here is, I believe, the 
point in which we disagree. On 
referring to page 233 of your memoir, 
diagram 5, if I have rightly under- 
stood your exposition, the thick line 
indicates the passage of the thick 
margin of the wing, the narrow line 
the passage of its thin border; these 
two lines are generated in one single 
movement of the wing, which in the 
case of diagram 5 is carried from left 
to right, as the direction of the arrows 
of the thick border, and when we 
place a little speck of gold on each of 
these two borders we see formed two 
figures of 8 parallel with each other, 
as I represent it here. [Fig. 1 of 
present communication.] I consider 
the changes of plane and all the 
deviations of the course of the wing 
as the effeCt of the resistance of the 
air, in which the wing is entirely 
passive. If my imperfecCt knowledge 
of the English language has not led 
me into error, the wing, according to 
you, would change its form actively, 
and would rotate on its articulation 
as if to screw itself in the air. Ex- 
periments which I have made by 
