370 Prof. Knoblauch on the Influence of Metals 
of gold, the perfection of the polish was not to be compared with 
that of the silver surface, for in optical respects the former did 
not cease to show its red, and the latter its yellow colour; while 
the polished silver and the metallic mercury, colourless in them- 
selves, gave only the colours of the mirrored objects. With the 
former, therefore, the heat reflected under a definite angle was 
not purified from the diffuse heat ; in the case of the latter alone 
was the pure and simple reflexion present.” 
The dependence of the diffuse reflexion on the inclination of 
the incident calorific rays to the reflecting surface remains now 
to be examined ; and here, in the first place, we have to ascertain 
the quantity of heat which reaches the thermo-electric pile under 
different angles. The metal plate to be tested was placed in the 
path of the rays sent in from the mirror of the heliostat, and an 
arrangement was made which permitted the plate to be turned 
round a vertical axis, so as to permit the rays to fall upon it at 
any required angle. The surface of the thermo-electric pile was 
placed at right angles to a line which made the same angle with 
the reflecting surface as the rays which fell upon it from the helio- 
stat. This arrangement permitted of an examination of all rays 
which fell upon the surface between the limits of 80 and of 2 
degrees. i 
A uniform dull plate of gold was first examined, and a gradual 
augmentation of heat was observed as the rays fell more and 
more parallel to the surface, Thus, for example, when the incli- 
nation of the surface to the rays was 2°, the quantity of heat 
sent to the pile was five times the quantity sent when the incli- 
nation was 80°. With another rough surface of gold an aug- 
mentation in the ratio of 1 to 3:5 was observed, under the 
same circumstances; with polished gold the augmentation was 
only in the ratio of 1 to 1°36. The gradual increase is exhi- 
bited in the following Table, which also renders manifest the 
difference between the rough and polished surfaces. 
Inclination. 80°, 60°. 40°. 20°. 10°. 2°, 
Rough surface .......s0e8 1:00 | 1:44 | 2:06 | 2:94 | 3°95 | 5:08 
Another rough surface ...| 1°00 | 1:29 | 159 | 2°59 | 3:00 | 3:50 
Polished surface..........+. 1:00 | 1:06 | 1:06 | 1:12 | 1:21 | 1:36 
Fine grooves were afterwards drawn on the metallic plates, 
and the effect of these plates when the grooves \.ere transverse, 
and when they were parallel to the plane of reflexion, was ex- 
amined. ‘The experiments were made with two pla‘es of gold, 
on which the fine grooves had different degrees of distinctness, 
The following are the results of the experiment :— 
