4.43 TRANSACTIONS OF SECTION A. 
Instead of an arrangement for producing a spectrum, the second instrument 
employs coloured glasses. The less homogeneous light which is furnished is of no 
disadvantage if we base the standardisation of the readings of the instrument on 
the black body. But if in the measurements made with the pyrometer one wishes to 
employ Wien’s law, for example, by which one somewhat extrapolates the tem- 
perature scale, then the effective wave-length must first of all be determined from 
the law of radiation by means of a special investigation. Determinations up to 
about 1600° indicate that the effective wave-length transmitted by the most 
common copper-oxide glass is constant, and is equal to ‘650,. 
From optical measurements based on the melting-point of gold (1064°) are 
obtained the following melting-points: Palladium, 1580°; platinum, 1790° ; 
rhodium, 2000°. 
The highest temperature which up till now has been determined with the gas- 
thermometer is that of palladium. The value obtained was 1575°. 
If we employ the optical pyrometer, which has been standardised by means of 
a black body, on a body of smaller radiating power, we do not obtain the true tem- 
perature, but a less value, which in distinction from the true temperature we call 
the black temperature. It is that temperature at which a black body will be if it 
radiates in the region of the spectrum which is investigated with the same bright- 
ness as the body of less radiating power. In many cases one can inclose the 
radiating body, if its emissive power is not known, in a cavity or in a reflecting 
envelope, and in this way make its radiation that of a black body. In other cases 
the emissive power must be specially determined. 
For the noble metals, such as silver, gold, and platinum, measurements have 
furnished a simple result, The emissive power (relative to that of a black body 
at the same temperature) for these metals for a given wave-length in the visible 
region is independent of the temperature within the accuracy of the measurements 
of temperature. Thus platinum emits red light of about one-third, gold of one- 
eighth, and silver of one-fourteenth of the brightness of a black body at the same 
temperature. The law is valid down to low temperatures where we can deter- 
mine the emissive power from the absorption of good metallic mirrors, 
2. Optical Pyrometry. By Professor C. Firy. 
Of all methods of temperature measurement those based upon the laws of 
tadiation are certainly the most exact, and they lend themselves readily to the 
construction of instruments which do not vary in their indications, as they are 
not subjected to the destructive influences of hot furnace-gases or of the substances 
under experiment. 
In the first place I should like to say a few words concerning the formule 
upon which temperature measurements may be based. 
I. The § Displacement Law’ of Wien 
Neary WERIC 
Ny, dee rN 
does not lend itself to exact measurement. This law may be expressed by saying 
that the product of the absolute temperature of a ‘black body’ and the intensity 
of radiation of the dominant wave-length is constant, 
