TO USE DIFFUSED DAYLIGHT 



411 



light ' with the simple mirror when no condenser is used is frequently 

 done in a most inaccurate manner. The correct method of doing 

 this is shown in fig. 351. F is the plane of the object, C is the con- 

 cave mirror, the mirror being placed at the distance of its principal 

 focus from the object. But the manner in which it is usually done, 

 from want of thought or knowledge, or both, is shown in fig. 352, 



FIG. 351. Illumination for ' diffused daylight.' 



where it is manifest that there is a total disregard of the true focal 

 point of the mirror and its incidence on the plane of the object. 

 From the impracticability of this diagram as a representation of a 

 working plan of illumination, we may see at once the importance of 

 having the mirror fixed upon a sliding tube, so that its focal point 

 may be adjusted 



It is also important here to note that in daylight illumination a 



FIG. 352. Erroneous method of arrangement for ' diffused daylight.' 



plane mirror gives a cone of illumination, as in fig. 353, when there 

 is ample sky -room ; but a window acts as a limiting diaphragm. 



In regard to the parallelism of the direct solar rays there is of 

 course no question. But the parallelism of that portion of the solar 

 light w r hich goes to form the firmament in our own higher atmo- 

 sphere is so completely broken up by refraction and reflexion 

 amongst the subtle particles of this higher atmosphere that the rays 



