384 SUMMARY OF CURRENT RESEARCHES RELATING TO 
Street. Instead of the usual lime-light an arc-lamp was the means of 
illumination, and many difficulties in the use of this light had to be 
overcome. 
Very high magnifications were not attempted, as the projection 
method is not adapted to give good results under these conditions. 
When projecting with an objective alone, this has to be brought 
very close to the slide, with high powers closer than the cover-glass will 
allow. In this case special substage condensers are necessary. This 
difficulty is more especially felt when the arc-light is employed instead 
of the lime-light. It can be surmounted either by the introduction of 
plano-concave lenses on the screen side, which have the effect of giving 
a greater working distance, or preferably by using an eye-piece. 
In the eye-piece method adopted by the author almost the exact 
conditions can be complied with for which the objective was made. 
Owing to the field not being flat, all parts of the objects cannot be 
brought into focus at once, but only successively by slightly shifting 
the focusing screw. It is only with very considerable depth of focus 
that for projection work over-correction for flatness can give a sharp 
picture, since without great care certain forms of distortion will be 
introduced. By stopping down the objective greater flatness may 
be secured, but only at the expense of light. 
The author exhibited various microscopic objects by projection under 
different magnifications. The screen distance was 21 ft. The lenses 
and magnifications employed were as follows : — 
First, a 35-mm. Zeiss projection objective, 4-in. substage condenser, 
Zeiss Huyghens eye-piece 2 ; 500 diameters = 250,000 times = penny 
stamp stretched to cover about 147 square yards. 
Second, a 1-in. Newton’s projection objective, 4-in. substage con- 
denser, Zeiss Huyghens eye-piece 2 ; 1000 diameters = 1,000,000 times 
= stamp stretched to about 588 square yards. 
Third, 1-in. Newton’s projection objective, 4 -in. substage condenser, 
Zeiss Huyghens eye-piece 3 ; 1300 diameters = 1,690,000 times = stamp 
stretched to about one-fifth of an acre. 
Fourth, 1/4-in. Zeiss’s achromatic objective, Abbe’s 3-lens substage 
condenser, with top lens removed, Zeiss Huyghens eye-piece 3 ; 4500 
diameters = 20,250,000 times = stamp extended to nearly 2J acres. 
With the polariscope various objects, such as glass in a condition of 
strain, Rupert’s drops (broken in the field), and mineral sections, were 
exhibited, both in parallel and convergent light. 
With the solidiscope, a new form of apparatus for exhibiting solids, 
and consisting of two reflecting prisms and suitable projecting lenses, 
were shown Barton’s button, the works of a watch, and a coin. 
A spectrum was projected upon the screen by a new method which, 
by means of a carbon disulphide prism combined with a reflecting prism 
or with a mirror, gives practically a direct spectrum without the 
necessity of turning the lantern. 
Electrical Thermostat.* — Mr. W. P. Kurtschinski has devised a 
thermostat with an electric regulator and heated by a mineral oil 
lamp. 
* Wratsch, 1892, p.744. See Zeitschr. f. wiss. Mikr., ix. (1893) pp.'473-4(l fig.) 
