ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
477 
In addition to this advantage of sure regulation and constancy of 
temperature, the object-stage admits of a rapid attainment of a desired 
temperature as well as of a rapid alteration. 
The attainment of a desired temperature (e.g. 37°) occurs diiring 
the passage of the main current. When the thermometer shows 37°, 
then the platinum thread is brought into contact with the quicksilver 
thread, and a distinct click is heard from the relay. The temperature 
then remains constant, and the main current, being shut off, is replaced by 
the auxiliary current. 
A higher or lower temperature is easily attained. To get a higher 
temperature, e.g. 45°, the platinum thread is withdrawn to a proper dis- 
tance by rotating the screw-head until the graduated thermometer shows 
45°. The quicksilver and the platinum threads in the contact thermo- 
meter meet ; the main current is shut off and the relay called into action 
as before. A reverse movement of the platinum thread combined with 
exclusion of the currents permits a cooling down somewhat below the 
desired temperature. Then by manipulation of the currents the tempe- 
rature is raised to the desired point. 
(5) Microscopical Optics and Manipulation. 
Zeiss’ Hew Comparison Spectroscope.* — This instrument is in- 
tended to assist botanists and physiologists in the study of such coloured 
substances as chlorophyll, and is so named because its special feature is 
to render possible an exact comparison of the absorption spectra of solu- 
tions. As will be seen from the accompanying figure (fig. 85), the appa- 
ratus has the general form of a Microscope, and so far resembles one that 
the special spectral arrangement can be applied to a Microscope stand. 
The object-table F bears two orifices, 4 cm. apart, through which two 
mirrors reflect the sun or lamplight perpendicularly upwards. Each of 
these two pencils, passing through prisms situated in the horizontal box, 
becomes decomposed and forms a spectrum. The two spectra appear in 
close proximity, and can be observed through the broad slit C in the 
eye-piece. 
In the side tube D is placed the wave-length scale, which is illumi- 
nated by the mirror at the tube’s mouth, its image being projected be- 
tween the two spectra. The width of the slit, and consequently the 
brightness of the spectra, can be altered by the turning of the knob A. 
Rotation of the tube-piece B brings out sharp presentation of the 
scale and spectra ; the screw E causes a side movement of the scale 
image. The two plane and concave mirrors are movable round vertical 
and horizontal axes. When the necessary mirror adjustments have 
been made, two spectra with the most important of Fraunhofer’s lines 
are seen over one another. It is easy by movement of the scale to get 
the D line on a = 589. When daylight is not available, a spirit-lamp 
with a salted wick or asbestos thread soaked in NaCl solution may be 
used. When direct sunlight is used, the number of Fraunhofer’s lines 
that appear is endless. The cross-piece carrying the prisms and super- 
incumbent ocular slides up and down in a vertical axis, and can be 
clamped at any desired height. 
* Bot. Centralbl., lxxiii. No. 10, pp. 349-52 (1 fig.). 
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