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*Appendix*.

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 10*l.* according to our former rules, but with it is worth 4*s.* lesse. Now you must remember that this 4*s.* must be lookd upon as the 50^{th} part of the value, and therfore you must abate 10*l.* in a stone of 500*l.* tho you abated but 4*s.* in a stone of 10*l.* Moreover suppose there be not only the black speck abovementioned but an icecle also in your stone of 10*l.* for which you are to abate 10*s.* and consequently the icecle & the speck 14*s.* Now I conceive that, because there are two faults, you must not only abate 10*s.* & 4*s.* but the double of the same, namely 28*s.* Again suppose that beside the speck and the icecle there be also a cloud, for which alone you might abate 6*s.* more, that is 4*s.*, 10*s.* & 6*s.*, in all 20*s.* I say that in this case you must not only abate barely 20*s,* 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 3*l.*, leaving your stone of 10*l.* reduced to 7*l.* Now this triple abatement in a stone of 500*l.* would be 150*l.*, because that 150*l.* is ^{3}⁄_{10} of 500*l.*, as the 3*l.* was ^{3}⁄_{10} of 10*l.*