In my radiometer the standards are composed of the Lovibond standard glasses in combination. The apparatus itself consists of an optical instrument or viewing box. This is divided by a central partition, so that on looking through the eyepiece one sees a white background through two small circular apertures. On one side, level with the background, is a fitting to take the pastille in its holder. On the other side is a groove in the instrument itself for the insertion of the standard glasses. A similar groove is fitted on the pastille side of the instrument to take neutral tints if required. The colour of the pastille as seen by reflected light can thus be compared with that obtained by transmitted light through the standard glasses seen against the white background. A difference of 1/5 B or 1 H is quite easily perceivable. It is usually of no great importance to obtain extremely accurate measurement of the smaller fractions below 1/3 B. Where this is necessary, neutral tints must be used when working with daylight. With electric light these are unnecessary. When required the neutral tints are interposed between the pastille and the eye to absorb the white light reflected from the pastille. The neutral glasses required are 1·5 for the unexposed pastille, 0·6 for 1/4 B, and 0·2 for 1/3 B. These values are subject to slight variations due to changes in the varnish of the pastille emulsion. The difficulty can always be avoided by using electric light, where a trace of neutral tint is needed only when matching the unexposed pastille--an unimportant point.