394 THE AMERICAN MONTHLY (Dee 
ming in the nymph stage. These again all turned out to 
be a species of the genus Hydrachna,~ By comparing the 
size of the red larvae found on Notonecta glauca in the 
autumn of 1898 with those found on the same form at 
the same spotin the spring of 1899, I have come to the 
conclusion that the water-mite larvae remain attached to 
their hosts fora whole season—namely, from the sum- 
mer of one year until the spring of the next—and the 
fact that those I found in the autumn did not undergo 
any alteration helps to strengthen this opinion. 
I have further found two small yellow pear-shaped lar- 
val water-mites on the larval form of a gnat,and also one 
on an Ephemera larva, but I have been unable to rear them. 
I have found also thatthe water-scorpion, Nepa cinerea, 
is a favorite host with some species. But I have not 
been able to keep any alive long enough to find out to 
what species of water-mite the larvae belong. InEpping 
Forest I took a specimen of Ranatra, literally covered 
with red water-mite parasites of all sizes. This I succeed- 
ed in keeping alive until ten specimens became free-swim- 
ming. They all turned out to be nymphs of Hydrachna 
globosa Geer. But the most curious find, perhaps, in 
this connection, has been a small fish from a pond on 
Earlswood Common with two larval forms of water-mites 
attached, which I take to belong to a species of Arrenurus. 
Quekett Club. 
Viewing Diffraction Spectra. 
J. REINBERG, F.R. M.S. 
It isa matter of common knowledge that the diffraction 
spectra, which an object under the microscope gives rise 
to, may be observed by removing the eyepiece and look- 
ing down the tube. But unless the eye is kept perfectly 
steady, which is difficult, they shift and change about. | 
For this reason Dr. Johnstone Stoney recommended look- 
