ZOOLOGY AND BOTANY, MICROSCOPY, ETO. 
475 
another so freely that, in the absence of any published account, it is 
difficult to determine either the originator of any particular improve- 
ment or the date of its introduction. As, however, Benjamin Martin 
was a mathematician, who understood both the theory and practice of 
optics, this invention may be safely credited to him. The body has a 
draw-tube. 
The date of this Microscope must be placed between 1759 when Martin 
came to London, and 1782 when he died. If, therefore, we say between 
1760 and 1770, we shall not be far wrong. It is very portable, the 
outside measure of its box being 10J X 7J X 2J in., and its weight in 
box with apparatus 4J lbs. 
The second Microscope (fig. 82) is by Cary ; it is a small simple 
Microscope of a common form, fitted with rack-and-pinion stage focussing. 
The vertical pillar screws into a brass plate, let into the box where the 
hasp is usually placed. 
It has also a compound body which may be used in conjunction 
with the simple lenses. This compound body, which is only 2f in. 
long, has a compound eye-lens, similar in arrangement to that of 
Martin’s described above. There are four powers, which screw into 
one another in the usual manner. The box measures only 3J x 3 
X 1J in. 
Electrically Heated and Regulated Warm Stage.* — Dr. Rudolf 
Kraus, after summarising the various heating stages that have been 
invented since the first (by Strieker in 1871), points out that no new 
application of electric methods has taken place since Stein’s in 1884. 
Stein’s method was to place in the hollow stage a spiral connected with 
Fig. 83. 
an electric current ; but this plan had the faults of deficient regulation 
and liability to great variations of temperature, due to the fact that 
air is not a good conductor of heat, and therefore is not adapted for 
a steady constant heating medium. 
The next idea was to make use of liquids as conducting media ; and 
after many attempts Herr Ehman has succeeded in finding one, viz. 
paraffin oil, which conducts heat and is not itself decomposed by the 
current. 
* Centralbl. Bakt. u. Par., xxiii. (1898) pp. 10-20 (2 figs.). 
2 K 
1898 
