REFERENCE TO HORTICULTURE. 43 



Light, to a certain extent, follows the same laws as heat. It is re- 

 ceived by radiation from the sun, reflected by smooth surfaces, trans- 

 mitted and refracted by transparent bodies, such as water and glass ; 

 concentrated by reflection from concave surfaces, and dispersed by re- 

 flection from surfaces which are convex. All these properties of light 

 are rendered more or less available in horticulture. Light, however, 

 differs from heat in the impossibility of retaining it after the absence 

 of the sun ; whereas heat can be retained by enclosing heated bodies 

 in non-conducting mediums, and by reflecting it back to the surfaces 

 troiu which it is radiated. 



The reflection of light is most direct when the impinging rays strike 

 the surface at a right angle, and least so when the angle is most oblique : 

 because, in the former case, the rays are reflected straight back, and 

 consequently the objects in front are illuminated proportionately ; 

 but in the latter case the greater number of rays pass off at one side, 

 and illuminate less effectively the objects in front. The reflected 

 rays are always returned from the surface on which they fall, at 

 an angle equal to the angle of incidence ; if the reflecting surface be ti 

 plane, the reflected rays will be parallel to each other : if the surface 

 be convex, they will be divergent, and consequently dispersed ; and, 

 if it is concave, they will be convergent, and hence concentrated. 

 Smooth and shining surfaces reflect most light, and rough and dark 

 surfaces least ; and, with respect to colour, white reflects almost all the 

 rays of light which fall on it, and black absorbs them all. 



When light falls on a transparent medium, a portion of the rays is 

 transmitted through it, and a portion is reflected from its surface. The 

 latter portion follows the same laws as the light which is reflected 

 from opaque surfaces ; and the portion which passes through it is re- 

 fracted that is, it leaves the transparent medium at a different angle 

 from that at which it fell upon it ; and by this change the light is also 

 weakened, so as at a very short distance from the surface of the trans- 

 mitting medium, as of glass for example, to be dispersed and trans- 

 fused in the atmosphere, in which state, in hothouses, it has no longer 

 the same power on the vital energies of plants. We are not aware 

 that the cause of the inefficiency of light, after it has passed through 

 glass and reached a certain distance, has been fully explained ; but the 

 fact is well known to gardeners, who, in hothouses, invariably place 

 the plants which they wish to thrive best at the shortest distance from 

 the glass. As the quantity of light which passes through glass at the 

 roof of hothouses is, all other circumstances being the same, greatest 

 when the plane of the roof is at right angles to the plane of the sun's 

 rays ; hence, the slope of the roof is, or ought to be, adjusted to the 

 direction of the sun's rays at that season of the year when its light is 

 most wanted. As in houses for early forcing, the greatest deficiency of 

 solar light is in the winter season, when the sun is low, so the roois of 

 such houses are made steep, in order that the sun's rays may be re- 

 ceived at a smaller angle. Summer forcing-houses, on the other hand, 

 have less steep roofs, so as to receive most benefit from the sun in 

 April, May, and June, when forced fruits are ripening. A greenhouse, 



