B. I heartily thanke you.
A. You must make such a measure upon your glass table as may correspond to the value of your grain, and when you have by the weight found how many grains you are to pay for, and by your note of colours at how much per grain, & when you have again by your table of magnitudes found how many squares you are to pay for at the same rate at which you reckoned the graine, then adding the value by weight to the value by extent, the half of that summ is the value of that stone according to its weight, extent & colour.
B. I apprehend. And I thinke there remains nothing more then to teach me how to make my abatements of the value so found as aforesaid according to the several natures & numbers of the defects.
A. Well, this I will doe. You must remember you were to keep by you such and so many stones as doe contain all the usual faults of diamonds with the quota parts of the value which for each defect is to be abated. As for example, suppose there be a black speck in a stone which without it were worth 10l. according to our former rules, but with it is worth 4s. lesse. Now you must remember that this 4s. must be lookd upon as the 50th part of the value, and therfore you must abate 10l. in a stone of 500l. tho you abated but 4s. in a stone of 10l. Moreover suppose there be not only the black speck abovementioned but an icecle also in your stone of 10l. for which you are to abate 10s. and consequently the icecle & the speck 14s. Now I conceive that, because there are two faults, you must not only abate 10s. & 4s. but the double of the same, namely 28s. Again suppose that beside the speck and the icecle there be also a cloud, for which alone you might abate 6s. more, that is 4s., 10s. & 6s., in all 20s. I say that in this case you must not only abate barely 20s, nor the double thereof as when there were but two faults, but because there are three faults, you must abate the treble of all three, which is 3l., leaving your stone of 10l. reduced to 7l. Now this triple abatement in a stone of 500l. would be 150l., because that 150l. is 3⁄10 of 500l., as the 3l. was 3⁄10 of 10l.