HYGIENIC CONDITIONS OF GLASSHOUSES 19 
earliest glasshouses appear as dimly lighted structures. 
In their construction a large amount of timber was ~ 
used, and the glass, of inferior quality, was cut into small 
panes, in the fitting together of which big overlaps were 
allowed. Gradually, as the importance of good lighting 
was better realized, there commenced an evolution in 
methods of building which to-day finds expression in 
our much-improved structures, and which must still 
continue if the best results are to be obtained. In the 
process of development less heavy and clumsy super- 
structures and larger glass of superior light-transmitting 
qualities were employed, with a consequent improvement 
in the health of the crops. While improvements in 
house construction have continued to make steady pro- 
gress, it is unfortunate that so little attention has been 
given in the past by scientific investigators to glasshouse 
construction in relation to light intensity. Important 
contributions, however, have been made recently by 
Stone (48), whose work clearly shows the immense 
importance of further study along these lines. 
His investigations were concerned with a study of 
light intensities in glasshouses of different. construction 
and under different conditions, and while important 
results were obtained, the ultimate proof of the superiority 
of any given type of house will depend upon direct 
evidence obtained from a study of crops grown in such 
houses. Unfortunately these investigations cost money, 
which, in these days of strict economy, is difficult to 
obtain; but the ultimate end of such investigations 
must lead to information of great economic importance, 
far outweighing in money value the comparatively small 
sum required to gain it. 
Stone showed that morning light is more intense than 
afternoon light, his experiments giving a difference of 
intensity varying from 10 per cent to 30 per cent, in 
accordance with the time of the year. These results 
indicate that the orientation of glasshouses in relation 
to a north and south aspect has an important effect upon 
