198 PEACTICAL PHOTO-MICKOGKAPHY 



To ascertain the dimensions of a microscopic object, the 

 photographic method is probably the most accurate. If the 

 magnification is first exactly determined it is only necessary 

 to carefully measure the size of the object as seen in the photo- 

 graph, and then to divide the measurement so obtained by the 

 magnification. The result gives the actual size. With regular 

 recurring structure, such as in diatoms, it is best to measure 

 the distance covered by a definite number of markings for 

 example, ten and to divide the result again by this number. 

 Any error that there might otherwise be in taking a measure- 

 ment between the exact centre of any two lines or spaces is 

 by this means considerably reduced. 



The Dark-room : Its Illumination and Appointments. 

 The proper equipment of the dark-room is a matter of some 

 importance, although it will generally happen that some pro- 

 vision in this direction has been previously made ; it will 

 therefore be a question of utilising what already exists rather 

 than installing an entirely new equipment. 



The main questions in fitting up a dark-room are its 

 illumination, water-supply, ventilation, and regulation of 

 temperature. 



The illumination is the chief point, as the method of effect- 

 ing this will greatly influence the photographic results, and it 

 must be largely determined by the class of work to be 

 carried out. The usual source of light is what is known as a 

 ruby lamp, this being some ordinary white light of low intensity 

 such as an oil lamp, gas, or electric light which is enclosed 

 in a light-tight box, the latter having in one or more sides of it 

 a piece of ruby glass. As in photo-micrographic work plates 

 are used which are sensitive to a considerable range of the 

 spectrum, an ordinary ruby glass is not sufficient ; in fact, 

 most ruby glasses that are sold for photographic purposes are 

 not by any means safe even with ordinary plates, especially 

 if these be highly sensitive. No plate is entirely insensitive 

 to any portion of the visible spectrum ; it only requires to be 

 exposed to any source of light for a sufficient time for an effect 

 to be produced. It will be advisable to have one thickness of 

 rather deep ruby glass and to supplement this with either another 

 piece of a deep orange colour or preferably a piece of ruby 

 with one surface ground. It is usually better to have some 



