log_{10}J = K_{1} + K_{2}(1/T) (2) where K_{1} = log _c_{1}_-5 log λ and K_{2} = _c_{2}_(log _e_/λ). This simplified expression shows a linear relation between log J and 1/T; and hence if the temperatures corresponding to two intensities be known, the results may be plotted on squared paper in the form of a straight line connecting T and J, from which line intermediate or extraneous readings of temperatures may be obtained for any given intensity. Another useful form of Wien’s equation, referring to the ratio of two intensities J_{1} and J_{2}, is as under:— ┌ ┐ J_{1} _c_{2} log _e_ │ 1 1 │ log ───── = ─────────────── │───── - ───── │ (3) J_{2} λ │ T_{2} T_{1}│ └ ┘ where T_{2} and T_{1} are the absolute temperatures corresponding to J_{2} and J_{1} The value of _c_{2}_ is 1450000, when λ is expressed in millionths of a centimetre. Evidently, if the ratio J_{1}/J_{2} and the value of _c_{2}_, λ, and T_{2} be known, T_{1} may be calculated. When λ is not known, as in the case of a piece of red glass for which its value has not been determined, two readings at known temperatures will establish the value of (_c_{2}_ log _e_)/λ, and all other results may then be calculated. Examples illustrating the application of the formula will now be given. _Example I._—A black body at an absolute temperature T_{1} is found to give twice the intensity observed at 1200° abs., the comparison being made with red glass transmitting wave-length 65 × 10^{-6} cms. To find the value of T_{1}. Applying values to formula (3) ┌ ┐ 1450000 │ 1 1 │ log 2 = ─────── log 2·7183 × │ ──── - ──── │ 65 │ 1200 T_{1 │ └ ┘ and ┌ ┐ 1450000 × 0·4343 │ 1 T_{1} │ 0·3010 = ──────────────── × │ ───── - ────── │ 65 │ 1200 1 │ └ ┘ from which T_{1} = 1237° abs. _Example II._—The intensity of the radiations from a black body at 2000° abs. are found to be equal to those from a given standard, taken as unity. To find the intensity at 3000 abs., compared with the same standard. λ = 65 × 10^{-6} cms. Applying in (3) as before, ┌ ┐ J_{1} 1450000 × 0·43435 │ 1 1 │ log ──── = ───────────────── × │ ───── - ──── │ 1 65 │ 2000 3000 │ └ ┘