50 Mr Buchanan , On a solar Calorimeter used in Egypt 
The inner radius of the annular disc is T52 inches, and the ampli- 
tude of the sector to be removed is 
q-4 _ *2 
360° ~ = 42-3°. 
34 
The numerical data just worked out and relating to the re- 
flector used are collected in the following table : 
No. of Mirror 
1 
2 
3 
Description of Mirror ...... 
Inner 
Middle 
Outer 
Inclination to the Axis ... 
i 
61° 51' 
45° 
34° 6' 
Inner Radius inches 
1-34 
3-0 
5-0 
Outer Radius „ 
R 
3-0 
5*0 
6-85 
i Width of Mirror... „ 
m 
1-88 
2*83 
3-31 
Outer Radius of \ 
flat band J ” 
M 
3-40 
7-07 
12-23 
Inner do „ 
M —m 
1-52 
4*24 
8-92 
Amplitude of Sector to bej 
removed J 
42-3° 
105-5° 
158-5° 
The condition that one of the mirrors should be inclined at an 
angle of 45° to the axis was suggested by the fact that this is the 
angle of greatest efficiency and by the consideration that it is an 
angle which is familiar in mechanical workshops, and is on that 
account perhaps more likely to be laid off accurately than another. 
There is, however, no particular advantage in making this re- 
striction, because in designing a series of mirrors for a reflector 
one of them is sure to be inclined at an angle of nearly 45° and to j 
have an efficiency which is sensibly the same as if the angle 
were 45°. 
The general problem, to construct the principal section of a 
reflector consisting of a series of conical mirrors when the direction 
and length of the common focal line are given and the position 
relatively to a point on this line, of a point occupying a definite 
position in the line of section of one of the mirrors is simple. 
The diagram, Fig. 6, shows a construction of this kind. 
The line OP is the axis of the reflector, it is also the direction 
of the incident rays when the instrument is in use. Make it the 
