240 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
however, do not in any way affect the accuracy of the measurement. In 
the first place, it is evident that as the beam is deflected the point h 
becomes more and more distant from the Microscope, and the pull on the 
fibre ceases to be vertical, but it must be also noticed that in doing so 
the scale D is carried out of the focus of the Microscope, which has in 
consequence to be adjusted by being moved forward to the exact amount 
which the scale had receded by the movement of the beam, and thus the 
arm A, carried by the end of the Microscope, is moved forward to an 
equal extent, the scale comes again into focus, and the fibre becomes 
again vertical. 
Again, in the case of the tube T being very long, it might happen 
that the spring of the tube and of the arm A might cause the fibre to 
appear more stretched than it really is, but the error due to this cause 
can be perfectly eliminated by finding, in the course of the experiment, 
the force that is being applied to the fibre, and afterwards placing 
w^eights on C until a pull of the same amount is obtained. As a matter 
of fact, however, with ordinary fibres the farther movement of D under 
these circumstances is not observable. 
Pol I, A. — Note di Microscopia. III., II condensator nei Microscopi. (The con- 
denser in microscopy.) 
llivista Scicnt.-Indnstr.^ XXI. (1889) Nos. 18, 19. p. 217. 
C4) Photomicrography. 
Bourdin’s Photomicrographic Apparatus.* — After describing 
Duboscq’s large Microscope for photomicrography,! M. J. Bourdin 
advises microscopists not to neglect the very simple method of producing 
jihotomicrographs by means of a 
Fig. 26. small camera, applied in the body- 
tube when the eye-piece is removed, 
large enough to permit the use of 
glass plates of 3 cm. square. The 
exposure of the sensitive plate is 
very short, and the magnification 
being low, it becomes necessary to 
employ an enlarging apparatus with 
which one may readily obtain trans- 
parent positives as large and as 
sharp as may be desired, especially 
by the use of Cowan’s chloro- 
bromide plates which ar 
with a solution of 30 
acid and 20 grm. carbonate of 
ammonia in 100 grm. distilled 
water. 
The small camera in question 
is shown in fig. 26, and is stated 
to be recommended by M. Luiz de Andrade Corvo, who is engaged on 
special investigations of the phylloxera. It is shown together with a 
focusing lens made by Starsnic, the successor of Verick. 
* La Liimicre Eloctrapie, xix. (1886) pp. 217-9 (2 figs.). f Supra, p. 231. 
J developed 
grm. citric 
