ILLUMINATION. 33 



ILLUMINATION. 



The best light for Photo- micrography is sunlight, 

 but the difficulties of its employment in our climate, 

 more than counterbalance the excellence of its re- 

 sults, and a description of the apparatus necessary 

 is only given for the benefit of those who can afford 

 sufficient time during daylight for Photo- micro- 

 graphy, or who may be more favourably situated 

 atmospherically. 



It should be borne in mind that the actinic power 

 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 

 greatest intensity being about 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 assis- 

 tant. 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 



