34 ILLUMINATION. 



of the sun is subject to considerable variations, due 

 to its elevation above the horizon ; this depends on 

 the hour of the day and time of the year, the great- 

 est intensity being nbout noon in midsummer, while 

 at the same hour in midwinter it is about one third 

 less. No fixed rule can be given for exposure, its 

 duration therefore must be left to the judgment and 

 experience of the operator. 



The rays of the sun may be thrown on the con- 

 denser of the microscope from a mirror moved by 

 hand. This is inconvenient and requires an assistant. 

 Some means of keeping the light in position is almost 

 a necessity, and for this purpose several instruments 

 have been invented, the cheapest being designed by 

 Stoney, and lately modified by Spencer of Dublin. 

 As all these instruments are expensive, we shall 

 describe a form of Heliostat very easily made and 

 sufficiently accurate for the purpose. 



A thick well-seasoned oak or mahogany board 

 about six inches wide and twelve long, A, fig. 1 7, is 

 fitted with levelling screws, xxxx, at its four corners. 

 On it a small French clock M is placed, a boxwood 

 wheel one inch in circumference being previously 

 attached to the hour hand axle. 



A sheet of white paper, with a line NS drawn 

 across it, is gummed to the back of a square of thick 

 plate glass, D, the whole being fastened to A, as in 

 the figure. 



A ball and socket joint is fixed at E, the ball 

 having a hole half way through it, the orifice being 



