38 ILLUMINATION. 



NS should not be disturbed. The position of the 

 needle should finally be tested by seeing that the 

 plumb line hangs directly over the N extremity of 

 NS. The silk cord is placed round the wheels on 

 the clock and needle, so that the mirror revolves in 

 the direction of the sun's motion when the clock is 

 set going. The sun's rays are directed on the mirror 

 L by the mirror K, and as L can be moved in all 

 directions, the reflected rays may be made to occupy 

 any position. 



This instrument has been described for the benefit 

 of those who haVo the time and ability for the manu- 

 facture of their own instruments, and because it 

 cannot be purchased ready made. It was found 

 sufficiently accurate for exposures of moderate 

 length, but like all instruments of the kind it re- 

 quires considerable care and practice to adjust. 

 When once fixed, its position should be left un- 

 changed, as a separate adjustment every time it is 

 required will be found very troublesome. 



Stoney's heliostat can be purchased for five 

 pounds. It is a very reliable instrument and pos- 

 sesses the great advantage of only once reflecting 

 the incident ray. 



Artificial ligiit has so many advantages over sun- 

 light that its employment is almost universal in 

 micro-photography, Swan's electric lamp is the best 

 form, but as an ordinary paraffin lamp gives excellent 

 results, and the arrangements are similar for both, 

 we will confine ourselves to a description of the latter. 



