provided with lamps having flat wicks turned endwise to the condensers. Here again we find a divergence of opinion, some preferring two-wick, some three-wick, others four-wick lamps ; generally, however, three-wick lamps are used. We have used, with good results, however, a round-wick lamp, or so-called Argand burner, this being actually a Defries lamp of forty-candle power. The burner of this lamp is circular, with an air passage up the centre, and has a chimney of glass contracted just above the burner, and above the contraction the flame appears intensely luminous and solid ; it is at this point that the flame should be used. For this purpose it is necessary to provide the lamp with an outer case of brass or tin, which may be fitted in position and slid up and down, without in any way touching the glass chimney. The tinned sheet-iron is carried up above the glass chimney for some distance so as to lengthen the chimney, thus creating more draught, therefore more perfect combustion of the oil and a more actinic light. The outer sheet-iron case is provided with an aperture, circular, of half an inch diameter; and, on looking into this, nothing but an intensely luminous circle of white flame is seen. No matter what lamp is used, the circle of illumination thrown by the objective should show no lines of variable illumina-tion. The most important thing in all illumination is to have the radiant a point, otherwise we are troubled with parallax, varying illumination, and want of sharpness ; but, provided the degree of amplification be not too great, this trouble will not arise.
Gas and Enriched Gases. Ordinary gas, unless of good quality, is not so suitable as gas enriched by the vapour of some hydro-carbon. The commercial form of albo-carbon will be found very suitable. Mr. Traill Taylor has suggested a very convenient arrange-ment, M which consists of two fish-tail burners separated from each other by the extent of an inch, both flames having their flat sides towards the condensers, there being an opaque disc, with a circular aperture in it of a little over half an inch in diameter, placed as close as possible up against the foremost flame so as to reduce its effective area. The position of this aperture must be such as to be opposite to the most luminous part of the flame. The second flame behind the anterior one serves to confer in-tensity, and is of great utility; but nothing seems to be gained by a third burner. The gas flame, when thus enriched by the so-called albo-carbon (naphthalene), is very intense. An Argand