190 
One of our Common Monads. 
power definition that I here commend it. With its aid, objectives 
incapable of resolving certain difficult tests (such as Amphipleura 
pellucida, Grammatophora subtilissima, &c.) with white light, show 
them in a satisfactory manner, and those which even with white 
light are capable of displaying the most difficult tests, exhibit them 
with greater clearness and distinctness. I attribute this result 
chiefly to the well-known fact that the chromatic correction of our 
very best modern objectives is far from perfect, more or less of a 
secondary spectrum being always visible, and interfering with 
distinct vision. Moreover, many of the objects we desire to examine 
are themselves capable of producing enough chromatic dispersion to 
interfere with our perception of their true form. Both these evils 
are escaped by the method here described. I do not advise it as a 
substitute for other modes of using the microscope, but as a special 
means of research to be reserved for occasional use in connection 
with the higher powers of the instrument. 
I have frequently been asked to express an opinion as to whether 
the use of monochromatic sunlight is likely to prove injurious to 
the eye of the observer. On this subject I can speak from an 
extensive personal experience in connection with photo -micrography. 
The only injury to my own eyesight of which I have ever been 
conscious was produced by an injudicious exposure to the electric 
lamp. If the microscopist so manages his illuminating apparatus 
that the field of the microscope resembles in colour and intensity 
the azure blue of the sky on a clear day (and this is the condition 
which should always be aimed at), I do not believe the use of 
the method for any reasonable time will be found injurious. I 
have recently found, when a sheet of plate glass backed with 
black velvet is substituted for the ordinary plane mirror in any 
of the above arrangements, that while the brilliancy of the light 
is much moderated, its desirable qualities are unchanged, and it 
is still intense enough for the adequate illuminating of the 
highest powers. Those who find the light obtained from the 
ordinary mirror too brilliant may resort to this contrivance with 
advantage. — Paper read before the Philosophical Society of 
Washington, U.S.A. 
VII. — One of our Common Monads. 
By Professor Albert H. Tuttle. 
Since the investigations of Clark, Carter, and others upon the 
sponges and their allies, anything which adds to our knowledge of 
the Flagellate Infusoria is of especial interest. I have fortunately 
had an opportunity of making a careful study of a genus Urella of 
