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The familiar use of the subject of Geometrical white Optics by all who have received any but a very slight pure mathematical training precludes the possibility of idt originality in the facts themselves, and admits of
but small originality in their treatment. At the
this book is not merely a rescript of any existing 7 their work,
It might be thought that it would be an easy task to adapt a subject of simple principles to the comprehension of boys possessing mathematical knowledge of the standard usually found in the higher classes of
our schools. I imagined so myself, and during the first course of my working I have been gradually learning Oren my error. I can only wish that what I consider to dy be an object of the highest importance had been it approached by some one more fitted both by matheEter
matical knowledge and experience than by one whose the
sole qualifications are considerable fondness of the or subject and a great desire to afford to beginners some
glimpse of one of the steps to science to which their
are the subjects of Illumination of Surfaces and Achromatism.
I need not say that I shall feel most thankful for notices of corrections that are needed, and for any suggestions as to the improvement of the book.
April 2, 1870.
Page 17, fig. 12, lowest line from Q is unnecessary.
line 5 from foot, for fig. 29 read fig. 28.
OR line 6, for
read AQ-AO Aq - AO line 4 from foot, for AQ read Aq. 59, line 2 below fig., for fig. 38 read fig. 37; and in fig. insert (i)
to the upper, (ii) to the lower portion.
(iii), for S read S".
line 2 from foot, for 52 read 51.
f+CQ 90, line 10, for PCP1 read PC1P1 94, line 5, for fig. 53 read fig. 54. 95, line 6 from foot, for C, read Ci. I 20, line 2, for SE read vE.
85, line 3s for ft ce
it co read
THERE are certain ideas connected with light and vision which are familiar to us all from the moment that our powers of observation exist. We know, for instance, that some bodies send out light of their own, such as the sun, or a gas-flame, or a red-hot coal; whereas others send out only light which they have received from something else, as, for instance, the bright spots on the polished back of a chair which come from the light of the gas-flame, or the fire, or the moon, who sends us the light which she gets from the sun.
Then again, we cannot but feel sure that light comes from the object to our eye in straight lines. As an illustration of this there is the fact that if a small round hole be made in a sheet of paper, and a steady candleflame held before it, the light which passes through the hole will make an image of the flame upside down on another sheet of paper held parallel to the first. But this is only true so long as we suppose the light to be passing