562 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
The radii, &c. are expressed in mm. The kinds of glass are deter- 
mined by the refractive indices for the line D ( n D) of the solar spectrum, 
and for the line of the hydrogen spectrum ( H 8[ n G 1 J). The focal di- 
stances for rays parallel to the axis in different zones are : — 
For central rays {f D l = 223*275} A = “ °' 210 
For middle zone, F : 12 {f D 1 = 221*36o} A = + 0-040 
For marginal zone, F:8 (fD 1 ^ 223*442} A = + 0*053 
(3) Illuminating 1 and other Apparatus. 
New Thermometer for Regulating the Temperature of Paraffin 
Baths, &c.* — Most instruments for obtaining a constant temperature 
are liable to get out of order, so that it is advisable to use with them 
an apparatus which shall give warning when the temperature has risen 
above the desired degree. Such an apparatus is the contact-thermometer, 
which consists of an ordinary glass thermometer armed with two platinum 
wires. One of these wires is in contact with the mercury in the reservoir, 
while the other is fused in the tube at the point 50° C., so that the 
mercury touches the wire when it rises to this point. When this 
happens, an electric circuit containing a bell is closed, and the bell will 
continue ringing until, by regulating the flame, the temperature again 
falls. 
New Rotating Disc for the Preparation of Lacquer Rings.f — 
Mr. C. F. Betting has devised a new form of apparatus for making 
Fig. 88. 
lacquer rings. As seen in fig. 88, it consists of a rotating disc wdiicli is 
set in motion by a toothed wheel engaging in a pinion on the axis of 
* Zeitschr. f. ang. Mikr., ii. (1896) pp. 35- G. f Tom. cit., pp. 33-4. 
