CH. VI] 



DO AND DO NOT WITH SUNLIGHT 



165 



7. One mirror is attached to 

 the shaft and is driven by the 

 clock-work. The other mirror 

 is set by hand at the beginning 

 of the experiment ( 239, 248). 



8. As the rays of sunlight are 

 practically parallel, only one 

 element of the condenser is 

 needed, viz., the one next the 

 lantern slide (fig. 74, 256). 



9. To increase the illumina- 

 tion use a larger mirror and 

 condenser ( 257). 



10. To turn the light on and 

 off, use a metal shield (262). 



n. Use a heliostat designed 

 for the hemisphere where you 

 are to work (249-255). 



8. Do not use a condenser 

 with two or three lenses for sun- 

 light as for a near light source, 

 use only one lens or an achroma- 

 tic combination designed for 

 parallel light (fig. 74). 



9. One cannot increase the 

 illumination without increasing 

 the size of the mirror and con- 

 denser. 



10. Do not use inflammable 

 shields to block the light. Use 

 a metal shield between the 

 mirror and condenser. 



1 1 . Do not try to use a helio- 

 stat in the southern hemisphere 

 which was constructed for use 

 in the northern hemisphere. 



