ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 535 
earth and in an opposite direction to the earth and with one-half its 
velocity, or making a complete revolution once in 48 hours. It may- 
be made from the works of a common clock, having a balance, in the 
following manner : — 
First remove the striking parts of the clock and procure three cog- 
wheels,* one having 10 cogs, one 40 cogs, and the third wheel any con- 
venient number. Now fasten the one having 10 cogs to the spindle of 
the hour-hand and in its place. Next, to carry the mirror, make a 
spindle about three inches in length and fasten the wheel of 40 cogs to 
it at such a place that when it is in its place in the framework of the 
clock it will be on a level with the wheel of 10 cogs, and then drill 
holes to receive it in the framework of the clock, taking great care to 
have it sit perpendicular to the frame when in place. Now attach the 
third wheel to the framework so that its cogs will match with the other 
two wheels and cause the spindle carrying the mirror to revolve in 
the same direction as the hands of the clock. 
A plane mirror may be attached to the spindle by a ball-and-socket 
joint or any convenient means. 
Now make a flat tripod base of iron with three levelling screws, and 
attach the clockwork to it by means of a hinge so that it can be ele- 
vated to correspond to the latitude of the place. 
To use the instrument, set it up with great care exactly north and 
south and elevate the axis carrying the mirror by means of a protractor 
and plumb to correspond to the co-latitude of the place, so that the 
axis points directly to the north star, and then adjust the mirror so as 
to reflect the light to the desired place. 
This simple apparatus, if well made, will answer every purpose of 
the more expensive heliostats and will practically keep a beam of sun- 
light in a constant direction for hours at a time. 
As reflection from a glass mirror is not perfect, it is better in 
practice to not reflect the light at an angle too acute to the surface 
of the mirror.” 
Photomicrographs by Gas-light-t — Dr. G. M. Sternberg advocates 
the use of gas-light as a satisfactory artificial light for photomicrographic 
work. The objections to the use of the oxy-hydrogen lime-light are the 
considerable expense attending it and the inconvenience resulting from 
the necessity of frequently renewing the gas-supply when much work 
has to be done. The electric light is also very expensive unless an 
electric plant is at hand, and even then it may not be available during 
the day. Admirable results have been obtained by the use of an oil- 
lamp ; but to photograph bacteria, &c., which have been stained, coloured 
screens must be used, and then, owing to the loss of light, the time of 
exposure must be considerably increased. 
Under these circumstances the author was induced in 1889, when 
preparing a report of the investigations which he had made on yellow 
fever in Cuba, to experiment with gas-light, and obtained very satis- 
factory results. 
The objective used was the 3 mm. oil-immersion apochromatic of 
* “ Grooved band- wheels. may also be used.” 
t Proc. Amer. Micr. Sue., xiv. (1893) pp. 85-90. 
