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interfaces of flying clouds ; and yet, from the nature 
of this micrometer, they may be very fafely relied 
upon : though it would have been impoflible to have 
catch’d any one of them with the common microme- 
ter. 
The principle on which this moil excellent inftru- 
ment is conftruCted, was laid before this Society laft 
May: and it is to be hoped, .that Mr. Dollond will 
evince the certainty of its meafurements, from the 
lead: to the. greateft angle it is capable of comprehend- 
ing ; and that, under every confideration of reflexion as 
well as refraction by fphet ical furfaces ; fo as to leave 
no room for fuch objections or cavils, as otherwife may 
probably be brought againft it. For our own parts, 
we are fully fatisfied of the juflnef^ mereof, from a 
great variety of trials and comparifons. That which 
we have hitherto ufed, is the firfl: that has been 
made of the kind ; and might, perhaps, have been 
better conftruCted in fome refpeCts, tho’ in nothing 
material. 
Applied to a refleCtor of only two feet, the fcale 
is as large as the common micrometer can have 
in a forty-foot refraCtor; and all is done without the 
help of ferews or wires ; fo that there is no need of 
illuminating. 
In virtue of fuch a fcale it is, that even fractions of 
feconds may be depended upon; as we have found, 
by often repeated trials on the diameters of the pla- 
nets. Thefe, as well as fmall diftances of ftars, may 
be meafured in all directions, with equal and almoft 
incredible facility, without a polar axis ; as well out 
of doors, in a rough wind, as within. 
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