214 
POPULAR SCIENCE REVIEW. 
and is provided with a mouth opening in the centre of the oval disk, and an 
anus opening not far from it on the side of the calyx. There is formed at 
this stage a large cavity which divides into two parts : the upper part, 
uniting the hollow tentacles at their base, forms the so-called circular canal ; 
while below it, and connecting with it, we have a large cavity forming the 
perivisceral cavity, a mode of development of the circular ring and of the 
perivisceral cavity totally unlike that observed in Ophiurans, Starfishes, 
Echini, and Holothurians. Metschnikoff compares the mode of development 
of the upper and lower cavity to analogous processes in the embryonic 
growth of Alcyonella and other Bryozoa ; he traces a striking similarity in 
the structure and position of the digestive organs and tentacles with similar 
organs of Bryozoa. 
Number of the Beads in Surirella gemma . — Dr. Pigott, in a paper in the 
“ Monthly Microscopical Journal” for March, says that the Surirella gemma , 
ordinarily a line object, gives up beautiful beads. In the photographs 
presented to him by Colonel Woodward he counted with a pocket lens 
thirty beads, and carefully inserted the points of a pair of fine compasses into 
the centre of the first and thirty-first bead so as to give the exact measure- 
ment of thirty beads. This he found to be on Photograph xvii. 
1 inch and or 1 /A 125. 
1000 
The power marked on the card was 3100 (probably by a clerical error), 
which gives 
83,000 beads per inch, nearly. 
This appears rather too great. Fortunately, he says, the Colonel has sent 
him two photographs of the same diatom. The larger is not quite 2£ times the 
size of the smaller. If, therefore, the magnifying power employed be as 
stated upon the smaller one, 1034, the second photograph is magnified not 
3100 times, but 
2200 times, 
which gives nearly 
59,000 beads per inch. 
The Manufacture of Object-glasses. — A discussion has been for some time 
going on in the “ Monthly Microscopical Journal” between Mr. Wenham, 
our highest English authority, and certain American microscopists, on this 
subject. Among other papers is one by Mr. E. Bicknell, in which, observing 
that Mr. Wenham says: “In such a difficult and complex arrangement as 
a high-power object-glass it is almost impossible for all the makers to 
work to the same magnifying standard,” he remarks : “ I would like to 
inquire where all the much-talked-of mathematical formulae are P Does he 
mean to intimate that the best opticians work by ‘ rule of thumb ? ’ or 
that it is guesswork P I have been supposing all the time since I became 
interested in the use of the microscope, and seeing such terms as ‘ index of 
refraction/ ‘ dispersion,’ ‘ light flint/ ‘heavy flint/ and ‘Faraday flint,’ 
‘ crown glass/ &c., that there were some mathematical formulae involved 
somewhere in the construction of object-glasses; possibly this statement 
by Mr. Wenham may account for the discrepancy between the nominal and 
the actual power of some object-glasses.” 
