1902] MICROSOOPICAL JOURNAL. 21 
the world. Yet, on examining with the microscope the 
scales of the wings of these mimetic butterflies, it will be 
found that, so far from the similarity in superficial ap- 
pearance being similarity in detail and pattern, there is 
a wide diversity in scale structure and arrangement. 
The scales vary not only in numbers, but in size, color, 
and form, and this is the case even where one inedible 
species mimics another. Mr. Harry Yates exhibited un- 
der a number of microscopes tow-nettings from various 
localities, chiefly Naples, Port Erin, and Stromness. The 
results of these gatherings, collectively termed plankton, 
he said differed very much according to locality and time 
of the year. The state of the weather may also account. 
for the presence of specially abundant species. The 
young stages of nearly all forms of marine life are to be 
found in floating condition, at the mercy of tides and cur- 
rents, thus spreading the species over wide areas. 
C. BAKER’s ENGINEERING MIcroscopE.—This instru- 
ment has been specially designed for the examination and 
photography of metals. It contains several distinctive 
characteristics. As it is intended for the examination of 
opaque objects exclusively, no-means of substage illumi- 
nation is provided, and even no stage aperture. Opaque 
objects to be examined with moderate powers can be il- 
luminated in the ordinary way by means of a bull’s-eye, 
a side reflector, or a lieberkuhn, but with high powers a- 
vertical illuminator must be used. This is simply a col- 
lar placed above the objective with a small hole at the 
side through which a strong light from the lamp is di- 
rected, the size of the hole being adjusted by means of a 
diaphragm. Within the collar is a disc of cover-glass 
set at an angle of 45°, which without materially inter- 
fering with the vision reflects the light down through the 
objective to the object and back again to the eye. In 
some illuminators of this type a prism covering half the 
