44 LIGHT, CONSIDERED WITH 



in which no fruit is ripened, but in which abundance of light is re- 

 quired all the year, has commonly perpendicular glass to receive a 

 maximum of light during winter ; and a sloping roof of glass at an 

 angle of 45 ; which is found favourable for the admission of light at 

 every season, as well as for throwing off rain, &c. This subject, how- 

 ever, will be more fully discussed when we treat of the construction of 

 hothouses. 



The light of the sun, after it has passed through the clouds, is re- 

 fracted, to a certain extent, in the same manner as when it passes 

 through glass or water ; and if plants were kept constantly under a 

 cloud, but at some distance from it, and if the space in which they 

 grew were enclosed by clouds on every side, we believe the effect on 

 the plants thus enclosed would not be materially different from that 

 produced by an enclosure of glass. In the open air, however, clouds 

 are not stationary ; and even where a succession of clouds covers 

 growing plants for several days together, the space on which the plants 

 grow is open on every side for the access of reflected and transfused 

 light. This prevents the etiolation and want of colour which are found 

 in plants in the back parts of hothouses having shed-roofs ; but which 

 are never found in nature, even on the north side of walls, except to 

 a very small extent. Hence plant structures which are enclosed by 

 glass on every side, and which are circular in the plan, are more likely 

 to produce an equalization in the growth and appearance of the plants 

 within, than such as have glass on one side, and a wall or opaque body 

 on the other. 



As an isolated body, such as a cone or small hill, disperses light 

 most extensively when the sun shines, so when the sun is obscured by 

 clouds the cone or hill receives most of the reflected light transfused in 

 the atmosphere, because it is exposed to the atmosphere on every side. 

 For the same reason the summits of all bodies in the free atmosphere 

 receive more light than their sides ; and hence the trees in dense forests, 

 and the thickly-standing corn plants in cultivated fields, continue to 

 grow and thrive though they receive little benefit from light, except 

 i'rom that which strikes on the tops of the plants. Hence the great 

 importance of perpendicular light to plants under glass, and the ad- 

 vantages of conical, dome-like, angular, or ridge-and-furrow roofs to 

 plant-structures ; because they receive from the atmosphere the trans- 

 fused light on every side. 



Though art is quite powerless to increase the sum total of solar 

 light, whether direct from the sun or transfused in the atmosphere, 

 yet it possesses a considerable degree of power in increasing the effi- 

 ciency on plants of such light as may be transfused in the atmosphere. 

 Thus, by spreading out the branches of a tree against a wall exposed 

 to the south, much more light as well as heat is brought to act upon 

 the leaves, than if the tree were a standard in the free air ; because, 

 in the latter case, there would be neither the benefit of the reflection 

 of the wall, nor that resulting from the circumstance of every leaf 

 being exposed to the direct influence of the sun's rays when it shone. 

 In like manner, herbaceous plants or shrubs may be planted or trained 



