Having settled upon a standard candle or lamp the measurement of light demands extreme care, and, at the best, can never approach the accuracy of other laboratory measurements. Many photometers have been invented, some of them highly elaborate, but the type oftenest used remains in essence the simple instrument long ago devised by Bunsen. On a frame supported by a stand, S, is stretched a sheet of white paper in the centre of which is a grease spot. This spot allows more light to pass through it and consequently reflects less than the unmarked portion of the paper. If the sheet is more strongly lighted from behind than from in front, it appears bright on a dark ground. If it is illuminated more strongly in front than at the back it will seem dark upon a bright ground. When equal lights fall on both sides, the spot becomes invisible, since it can then appear neither darker nor brighter than the surrounding paper. In its simplest use the screen is placed between a standard candle or lamp at A and the light to be measured at B: the screen is moved along its graduated slide until the grease spot vanishes. If the screen is twice as far from B as from A when the spot disappears, then B is four times as intense as A in light; if the screen were thrice as far from B as from A, then B would be nine-fold as bright as A, the intensity of light diminishing as the square of the distance of its source. An open-arc lamp, without a reflector, sends to the ground a fairly wide ring of brilliant rays; on both sides of that ring the illumination is feeble. Other sources of light also vary a good deal in the brilliancy of the beams which they emit in various planes. It is therefore usual to measure the light from a lamp as sent forth in all planes, or at least in its principal planes. When incandescent lamps are brought to a photometer they are as a rule placed on a spindle turning so swiftly that their mean horizontal candle-power may be read at once. For measuring the mean spherical intensity a photometer devised by Professor Matthews of Purdue University is employed. This apparatus has a series of mirrors arranged in a semicircle around a lamp, reflecting all the received light upon a single surface. Light may have great brilliancy and yet be undesirable from its color; we are all familiar with the havoc that gas light may play with hues of blossom and leaf that in sunshine are beautiful. Through ages untold the human eye has been seeing by rays from the sun, and from immemorial habit is best served by light of similar quality. A simple instrument, the spectrometer, casts upon a screen the spectrum from a mercury tube, a Nernst lamp, a Welsbach mantle, or other illuminant, and enables us to compare it with the spectrum of sunshine. Then, as in placing a light pink shade over a Welsbach mantle, we act on the intimations of analysis greatly to the relief of the eye.