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the gradual darkening of various parts of the view； which was the old stone fort in the rear of the school garden； with the trees； fence； &c。 I then became convinced of the practicability of producing beautiful solar pictures in this way； but； alas！ my picture vanished and with it； allno not allmy hopes。 With renewed determination I began again by studying the nature of the preparation； and came to the conclusion； that if I could destroy the part not acted upon by the light without injuring that which was so acted upon； I could save my pictures。 I then made a strong solution of sal。 soda I had in the house； and soaked my paper in it； and then washed it off in hot water； which perfectly fixed the view upon the paper。 This paper was very poor with thick spots； more absorbent than other parts； and consequently made dark shades in the picture where they should not have been； but it was enough to convince me that I had succeeded； and that at some future time； when I had the means and a more extensive knowledge of chemistry； I could apply myself to it again。 I have done so since； at various times； with perfect success； but in every instance laboring under adverse circumstances。〃
I have very recently learned； that； under the present patent laws of the United States； every foreign patentee is required to put his invention； or discovery； into practical use within eighteen months after taking out his papers； or otherwise forfeit his patent。 With regard to Mr。 Talbot's Calotype patent； this time has nearly； if not quite expired； and my countrymen are now at perfect liberty to appropriate the art if they feel disposed。 From the statement of Mr。 Wattles； it will be perceived that this can be done without dishonor； as in the first instance Mr。 Talbot had no positive right to his patent。
Photography； or sun…painting is divided； according to the methods adopted for producing pictures； into
DAGUERREOTYPE； CHROMATYPE； CALOTYPE； ENERGIATYPE； CHRYSOTYPE； ANTHOTYPE and CYANOTYPE； AMPHITYPE。
CHAP。 II。
THE THEORY ON LIGHT。THE PHOTOGRAPHIC PRINCIPLE
Some philosophers contend that to the existence of light alone we owe the beautiful effects produced by the Photogenic art； while others give sufficient reasons for doubting the correctness of the assumption。 That the results are effected by a principle associated with light and not by the luminous principle itself； is the most probable conclusion。 The importance of a knowledge of this fact becomes most essential in practice； as will presently be seen。 To this principle Mr。 Hunt gives the name of ENERGIA。
THE NATURE of Light is not wholly known； but it is generally believed to be matter； as in its motions it obeys the laws regulating matter。 So closely is it connected with heat and electricity that there can be little doubt of their all being but different modifications of the same substance。 I will not； however； enter into a statement of the various theories of Philosophers on this head； but content myself with that of Sir Isaac Newton； who supposed rays of light to consist of minute particles of matter； which are constantly emanating from luminous bodies and cause vision； as odoriferous particles； proceeding from certain bodies； cause smelling。
The effects of light upon other bodies； and how light is effected by them； involve some of the most important principles； which if properly understood by Daguerreotypists would enable them to improve and correct many of the practical operations in their art。 These effects we shall exhibit in this and the following chapters。 Before we enter on this subject it will be necessary to become familiar with the
DEFINITIONS of some of the terms used in the science of optics。
Luminous bodies are of two kinds； those which shine by their own light； and those which shine by reflected light。
Transparent bodies are such as permit rays of light to pass through them。
Translucent bodies permit light to pass faintly； but without representing the figure of objects seen through them。
Opaque bodies permit no light to pass through them； but reflect light。
A ray is a line of light。
A beam is a collection of parallel rays。
A pencil is a collection of converging； or diverging rays。
A medium is any space through which light passes。
Incident rays are those which fall upon the surface of a body。
Reflected rays are those which are thrown off from a body。
Parallel rays are such as proceed equally distant from each other through their whole course。
Converging rays are such as approach and tend to unite at any one point； as at b。 fig。 3。
Diverging rays are those which continue to recede from each other； as at e。 Fig。 3。
A Focus is that point at which converging rays meet。
MOTION OF LIGHTRays of light are thrown off from luminous bodies in every direction； but always in straight lines； which cross each other at every point； but the particles of which each ray consists are so minute that the rays do not appear to be impeded by each other。 A ray of light passing through an aperture into a dark room； proceeds in a straight line； a fact of which any one may be convinced by going into a darkened room and admiting light only through a small aperture。
Light also moves with great velocity； but becomes fainter as it recedes from the source from which it eminates； in other words； diverging rays of light diminish in intensity as the square of the distance increases。 For instance let a fig。 1； represent the luminous body from ＇hipho_1。gif＇ which light proceeds； and suppose three square boards； b。 c。 d。 severally one； four and sixteen square inches in size be placed； b one foot； c two feet； and d four feet from a； it will be perceived that the smallest board b will throw c into shadow； that is； obstruct all rays of light that would otherwise fall on c； and if b were removed c would in like manner hide the light from dNow； if b recieve as much light as would fall on c whose surface is four times as large； the light must be four times as powerful and sixteen times as powerful as that which would fall on the second and third boards； because the same quantity of light is diffused over a space four and sixteen times greater。 These same rays may be collected and their intensity again increased。
Rays of light are reflected from one surface to another； Refracted； or bent； as they pass from the surface of one transparent medium to another； and Inflected； or turned from their course； by the attraction of opaque bodies。 From the first we derive the principles on which mirrors are constructed； to the second we are indebted for the power of the lenses； and the blessings of sight；for the light acts upon the retina of the eye in the same manner as on the lens of a camera。 The latter has no important bearing upon our subject。
When a ray of light falls perpendicularly upon an opaque body； it is reflected bark in the same line in which it proceeds； in this case the reflected ray returns in the same path the incident ray traversed； but when a ray falls obliquely； it is reflected obliquely； that is； it is thrown off in opposite direction； and as far from the perpendicular as was the incident ray； as shown at Fig。 2；