30 DESIGN. 



acting in some degree like the triangular prism, partially decom- 

 poses or deranges the order of the rays. 



The theory of the transmission of light through transparent 

 bodies is derived from the well-known law in optics, that the 

 influence of the sun's rays on any surface, both in respect to 

 light and heat, is directly as the sine of the sun's altitude, 

 or, in other words, directly as its perpendicularity to that sur- 

 face. If the surface is transparent, the number of rays which 

 pass through the substance is governed by the same laws. 

 Thus, if one thousand rays fall perpendicularly upon a surface 

 of the best crown glass, the whole will pass through, excepting 

 about a fortieth part, which the impurities of even the finest 

 crystal, according to Bouquer, will exclude. But if these rays 

 fall at an incidental angle of 75°, two hundred and ninety-nine 

 rays, according to the same author, will be reflected. The inci- 

 dental ande, it will be recollected, is that contained between the 

 plane of the falling or impinging ray and a perpendicular to 

 the surface on which it falls.=^ 



In building a green-house or conservatory, then, light ought 

 to form the first point of importance, as success in plant culture 

 is entirely subservient to it, and we know full well, from experi- 

 ence, that no skill, however perfect, and no attention, however 

 zealous, will compensate for a deficiency of- light. Indeed, no 

 contingent or permanent advantage can justify, to the mind of 

 the experienced gardener, the adoption of one inch of opaque 

 material in the sides and roof of a horticultural building ; and 

 no part of the structure, from the side-shelves and upwards, 

 should be rendered opaque that can consistently be covered with 

 a material capable of admitting the rays of light. For pillars 

 and other appendages of strength, the material ought to be as 

 light as is consistent with strength and durability in the struc- 

 ture ; and, although we do not recommend such an erection 

 adjoining a dwelling-house, experience has taught that, both in 

 this country and in others, a mere shell of glass, so to speak, is 

 not only the cheapest, but also the best adapted for the artificial 

 culture of a 1 kinds of plants, both for fruiting and flowering ; 



* See Inclination of Hot-house Roofs. 



