874 PROCEEDINGS OF SOCIETIES. KSnSJun'eTffi* 
led liim to tlie conclusion tliat tlie pseudo-tracliese were in reality 
tubes, by which, when the fly was sucking blood or other fluids, they 
would be drawn up into the tubes, thence through the main channel 
passing into the fly's stomach. The observations which he had made 
were communicated to a meeting of gentlemen in 1837, at the house 
of Dr. Bowerbank, and he (the President) had very lately had a 
singular opportunity of confirming those observations, at the house of 
his friend Dr. Millar. 
He (the President) had been accustomed to prepare the proboscis 
for examination by squeezing the fly. The pressure caused the organ 
to protrude ; and tying it at the bottom with a thread of very fine 
silk, he put it on the slip of glass, and then under the binocular ; by 
this means obtaining a beautifully clear view of the organ. There 
was this drawback in the mode of preparation just described, for the 
object soon dried up. But the pressure incidental to Mr. Topping's 
plan was avoided, and the parts of the proboscis were seen quite in 
their natural position. 
His friend Dr. Millar, in anticipation of the paper of Mr. Suffolk, 
had collected some 200 or 300 flies. Two of these he (the President) 
had put into a fly-box with a little meat and blood. They imme- 
diately attacked the blood, and in a few seconds appropriated it. 
Then squeezing them as before described, and mounting them at once, 
he saw under the binocular every tube to be distinctly filled with 
blood. He wished to know whether, in Mr. Lowne's opinion, the 
pseudo-tracheae had a definitely tubular character. He believed they 
were tubes with a small aperture at their extremity which acted as a 
filter, preventing the ingress of anything that might be injurious to 
the fly while it was feeding itself. 
Mr. Lowne, in reply, stated that he quite concurred in the idea 
that the pseudo-tracheae were tubes, but with this qualification, viz. 
that they opened by means of numerous slits which extend the whole 
length of their under-sides. 
The President then said that at Mr. Lee's request he would describe 
a small but effective addition he had just made to the " kettledrum," 
or double hemispherical condenser. By the super-position of a third 
lens he had been able to show the rhomboides in a most satisfactory 
manner to his friend Dr. Millar under the y^th-inch object-glass, 
the markings being very distinctly visible over the whole surface of the 
valve, and appearing as black as jet. With the ^-inch power the object 
was also seen with great distinctness. 
The condenser consisted at present of two hemispherical lenses 
with an adjustable diaphragm between them for the necessary regula- 
tion of the intensity of the illuminating pencils. On the surface of 
the upper hemisphere he had placed a disc of tinfoil pierced with the 
usual apertures at right angles to each other for rectilineal markings. 
These apertures were made to coincide exactly with the apertures of 
the adjustable diaphragm. Then, upon the top of the upper hemi- 
sphere he placed a third lens of about two inches solar focus, the 
lens used being the field glass of the B eye-piece with its plain side 
uppermost. 
