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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
same time it is hoped that it will be found that

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
daily mathematics are leading. I have endeavoured
as much as possible to avoid the example of those
popular lecturers who explain difficulties by ignoring
them. But as the nature of my design necessitated
brevity, I have omitted entirely one or two portions
of the subject which I considered unnecessary to a
clear understanding of the rest, and which appear
to me better learnt at a more advanced stage,--such

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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.

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Page 17, fig. 12, lowest line from Q is unnecessary.
37, fig. 24 (ii), 9,' should be 4, }, and similarly on page 39-

40, fig. 25 (3 and 4), cross line of arrow slightly too long.
46, line 7, for O read Q.

line 5 from foot, for fig. 29 read fig. 28.
54, line 7 from foot, insert radius of bowl -
57, line 3, for Art. 39 (i) read Art. 35 (i).


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.
65, fig. 40 (ii), for S read S'.

(iii), for S read S".
66, (iii) line 2, for T read T'' and for N read V.
73, line 5, for 49 read 48.

line 2 from foot, for 52 read 51.
83, line 12, for AO read A01
line 18, for O read o.


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

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