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282 Annual Register 
Account of Books for 1772.
The History and present State of Discoveries relating to Vision, Light, and Colours. By Joseph Priestley, L.L.D.F.R.S.
The work upon electricity, formerly published by Dr. Priestley, has given the world a proof of the advantages arising from the plan of treating science historically. Nothing can be more agreeable than a view of the gradual progression of human industry; and the gradual unfolding of knowledge, from the first imperfect hints, to a full view of the whole scheme of nature. 
However, this method too strictly, pursued, migh, in some cases, prevent a district view of the system, which it endeavors to explain: natural philosophy might be sacrificed to its chronology. The author, therefore, frequently and properly departs from the strict chronological methods of treating his subjects; and thus preserves the great object of instruction, to which the entertainment of the reader ought always to be subordinate. 
It cannot be doubted that the completion of a work by one man singly, carries with it the advantage of an uniformity and harmony, which the joint labours of even the wisest must want; and the great industry, as well as the knowledge of Dr. Priestley, has shewn us in this volume, that the well-directed exertions of an individual, may leave us no room to regret that more labourers did not work in this vineyard. 
We can only join our wish to that of all the learned, that the Doctor may find such encouragement as will induce him to finish this great undertaking, of which the history of electricity and the opticks, makes but a lesser though a very valuable part. The extract we shall offer the reader, shall be a general summary of the doctrine concerning light. 

"The more we now of any branch of science, the less is the compass into which we are able to bring its principles, provided the facts from which they are inferred be numerous. Because, in an advanced state of knowledge, we are able to reduce more of the particular into general observations: whereas, in the infancy of a science, very observation is an independent fact; and, in delivering the principles of it, they must all be distinctly mentioned; so that though a selection may be made, a proper abridgement is impossible. 
Notwithstanding the vast additions that have been made to the science of opticks within the last hundred years, a judicious summary of the whole will be much
shorter
For the YEAR 1772. 233
shorter now, than it would have been a century ago, and yet I hope it is much larger than there will be any necessity of making it a century hence; as it may be presumed that, by that time, a connection will be traced between many facts, which now appear to be unconnected and independent of one another, and therefore require to be recited separately.
To be as concise as possible in delivering the elementary principles of the doctrine concerning light, I shall purposely omit the application of them to any of the phenomena of nature, though that be the chief object in all philosophical enquiries; it being my business at present, barely to recite the knowledge we have acquired of the laws of nature, as discovered by an attention to those appearances.
The observations that were made in the first part of the last period of this history will authorize us to take it for granted, that light consists of very minute particles of matter, emitted from luminous bodies. Some of these particles, falling upon other bodies, are reflected from them, in an angle equal to that of their incidence, while other particles enter the bodies; being either bent towards or from a perpendicular to the surface of the new medium, if the incidence be oblique to it. In general, rays of light, falling obliquely on any medium, are bent as if they were attracted by it, when it was a greater degree of density, or contains more of the inflammable principle, than the medium through which it was transmitted to it. More of the rays are reflected when they fall upon a body with a small degree of obliquity to its surface, and more of them are transmitted, or enter the body, when their incidence is nearer to the perpendicular.
The velocity with which light is emitted and reflected is the same; and so great, that it passes from the sun to the earth in the space of about eight minutes and twelve seconds. The velocity of light is supposed to be increased or diminished by refraction, in proportion to the degree in which the angle of the refraction is less, or larger than the angle of incidence.
Rays of light, emitted or reflected from bodies, enter the pupil of the eye, and are so refracted by the humours of it, at to be united, accurately, or nearly so, at the surface of the retina, or choroides, and so make images of objects, by means of which they are visible to us. When a beam of light is bent out of its course by refraction, all the rays of which it consists are not equally refracted, but some more and others less; and the colour which they are disposed to exhibit, is connected invariably with the degree of their refrangibility. The red-coloured rays are the least, and the violet the most refrangible; and the rest are more or less so, in proportion to their nearness to these, which are the extremes, in the following order; violet, indigo, blue, green, yellow, orange, red.
These colours, when they are separated as much as possible, are still contiguous, and all the shades of each colour have, likewise, their separate and invariable degrees of refrangibility. When they are separated by refraction, the extremes are removed from one another to such a distance, that they divide
the

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