Transactions of the odontological society of Great Britain . ence. After many trials a 50per cent, solution was found to suit our purposebest. Before proceeding further I would like to givea short account of the principle upon which themeasurement is based. As, however, it hasbeen so exhaustively described by Oliver in theCroonian Lecture of 1895 no more than a mereabstract is necessary. The colour metric systemis used in various ways as a means of quantita-tive estimation with the aid of colour. Lovibond,the inventor, has fixed an arbitrary and equiva-lent unit in glass tinted in the three pr
Transactions of the odontological society of Great Britain . ence. After many trials a 50per cent, solution was found to suit our purposebest. Before proceeding further I would like to givea short account of the principle upon which themeasurement is based. As, however, it hasbeen so exhaustively described by Oliver in theCroonian Lecture of 1895 no more than a mereabstract is necessary. The colour metric systemis used in various ways as a means of quantita-tive estimation with the aid of colour. Lovibond,the inventor, has fixed an arbitrary and equiva-lent unit in glass tinted in the three primarycolours, the equivalence of the unit beingestablished by the fact that when the corre-sponding units of the three primary colours aresuperposed, the colour of each is destroyed, aneutral tint resulting. It is most interesting tonote the total abolition of colour produced bythe superposition of even the lightest shades of I THE SULPHO-CYANIDE OF POTASSIUM IN SALIVA 59 the domiDant colours. The latter are sub-divided into shades and arbitrarily numbered. in series, the shade of one colour correspondingto the number of the shade of the other two 60 THE SULPHO-CYANIDE OF POTASSIUM IN SALIVA colours. There are necessarily more light shadesthan dark ones, the human retina only beingable to see minute distinctions of colour in lightshades. For matching and testing experts areemployed. The chart is a diagrammatic representationof the colours of known solutions of givenquantities of sulpho-cyanide, to which a fixedamount of a 50 per cent, solution of neutralferric perchloride has been added. It is calledthe specific colour curve. The numbersbelow the base line are the percentage amountsof sulpho-cyanide sent for examination, whilstthose on the left correspond to Lovibonds ownstandards of colour. The curve itself is formedby a line uniting the points where the colours ofLovibonds units cross the lines of the sulpho-cyanide solutions. It will be seen, on lookingat the chart, that in
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