1901] MICROSCOPICAL JOURNAL. 293 
does contain certain forms (different from the “‘microbes’’) 
all microscopists seem agreed that water we drink con- 
tains no forms at all. If any scientist, doctor or amateur 
has ever seen any of these forms under 5,000 times mag- 
nification we shall be pleased to assist Mr, Walters in his 
business. He has not yet informed us when and where 
he saw them. 
Notes on Microscopy. 
M. I, CROSS. 
THE HELIOSTAT IN MicRoscoPpy.—They who have never 
seen the microscope used in conjunction with a heliostat 
have yet to learn the possibilities and advantages of = 
a form of illumination. 7 
Residents in bad climates have substantial excuses for 
not using the heliostat because of the uncertainty of get- 
ting bright sunshine at the time of working; still the in- 
termittent occasions on which it could be employed well 
repay them for their trouble. 
Microscopists with leisure would find it more useful 
than those who have few opportunities of working in the 
day time, while those who live in countries where con- 
tinuous sunshine can be depended upon are strongly rec- 
ommended to make trials of a heliostat. 
Its advantage is especially indicated in the illumina- 
tion by monochromatic light, for which purpose two 
right angle prisms of dense glass with about 14 inch face 
are necessary, with half a rapid rectilinear lens to con- 
centrate the light upon the prisms and the remaining 
half placed to receive the spectrum as it emerges from 
the prisms, : 
The microscope should be placed at some distance, say 
12 to 15 feet, from the prisms, the height of the heliostat 
being so arranged that the beam from it shall fall upon 
the microscope mirror. A strip of card should be set in 
front of the microscope on a level with the mirror, and 
