SCIENTIFIC SUMMARY. 
9 5 
The apparatus for measuring deflection consisted of a slide-rest, carrying 
an adjustable slit, and an achromatic eye-piece, with a single silk fibre in its 
focus. A piece of plane glass at an angle of 45° is next the eye. The eye- 
piece was moved till the fibre bisected the deflected image of the slit. 
To regulate and measure the speed of rotation, a tuning-fork, bearing on 
one prong a steel mirror, was employed. This was kept in vibration by 
means of electricity. It was so placed that the light from the revolving 
mirror was reflected to the piece of plane glass in the eye-piece and thence 
into the eye. When fork and revolving mirror are both at rest, the eye 
sees an image of the revolving mirror ; when the fork vibrates this image 
is drawn out into a band of light. When the mirror begins to revolve 
this band breaks up into a number of moving images of the mirror ; and 
when finally the mirror makes as many turns as the fork makes vibrations, 
these images are reduced to one which is stationary. Hence to make the 
mirror execute a given number of turns, it is only necessary to open the 
valve, until the images of the revolving mirror come to rest. The electric 
fork was compared with a standard Tit 3 fork, the temperature being noted. 
The beats were counted for 60". 
The lens was 8 in. in diameter, not achromatic ; focus, 150 ft. The sta- 
tionary mirror was 7 in. in diameter, silvered on the front surface. The dis- 
tances between the various parts of the apparatus having been determined, 
the heliostat and distant mirror were adjusted, and the electric fork connected 
against the standard for 60". The revolving mirror was then started, and 
regulated until the image came to rest near the cross-hair. The screw was 
then turned until the I’cross-hair bisected the deflected image of the slit. 
This was repeated until ten observations were taken, and then the tempera- 
ture was again taken, and the beats noted. Usually five sets of observations 
were taken morning and evening. 
The distance between the two mirrors was taken on au average of five 
measurements, and found to be 1986*23. The rate of vibration of the 
standard fork was found by allowing it to trace its record on the lamp- 
blacked cylinder of a chronoscope, time being given by a sidereal break- 
circuit chronometer and a fluhmkorff coil. 
The direction of rotation was at first right-handed, and afterwards 
reversed. 
The effects of the vortex of air about the mirror could be found at any 
speed. To prevent bias in observation, readings were taken by different 
observers independently of each other. 
The result obtained for the velocity of light, in vacuo , was 299*828 
kilometres per second. Foucault’s determination in C.G.S. units being 
2*98 x lO.io or 298 kilometres per second. 
ZOOLOGY. 
Development of Oystei's . — According to the generally received opinion, 
the eggs of the oyster are fertilized inside the shell of the parent, within the 
mantle-cavity of which the young are carried until they are provided with 
