92 PKACTICAL PHOTO-MICROGRAPHY 



lights, as the result of rise of temperature. Spectroscopically, 

 the light consists mainly of three bright lines one each in the 

 yellow, green, and blue- violet regions. It also has a very 

 extended ultra-violet spectrum ; but for the purpose under 

 consideration this is not of moment. It is obvious, how- 

 ever, that it is only necessary to screen off the colours which 

 are not required with suitable coloured screens, by a method 

 to be described later, in order to obtain a light which is truly 

 monochromatic, and which is, or may be, of one wave-length. 

 Even without any colour-screen it is optically more efficient than 

 other illuminants, since owing to the absence of red radiations 

 the mean wave-length is less. Consequently, the denning and 

 resolving power of any microscopic objective is appreciably 

 increased. 



The type of mercury-vapour lamp supplied commercially 

 for ordinary photographic purposes is not the most suitable 

 for microscopic work, as the containing-tube is far too large. 

 The lamp shown in the illustration (Fig. 27), being very much 

 smaller in size and having a glass tube of much less sectional 

 area, has proved to be entirely satisfactory. The current-con- 

 sumption is small, and with a glass tube should not exceed 

 one and a half amperes ; so that it is a source of light that can 

 be used with absolute safety on any house-lighting circuit, 

 current being obtained from an ordinary wall-plug or lamp- 

 socket. It has the additional and by no means unimportant 

 advantage, that there is practically no radiant heat ; hence, 

 if required, the lamp can be run in very much closer proximity 

 to the microscope than is possible with any other light-source 

 yet described. No water-cell to absorb heat need be placed 

 between the illuminant and the microscope. 



As the mercury is enclosed in a glass tube, exhausted to 

 such a degree that its vacuum is considerably higher than that 

 of an ordinary incandescent electric lamp, no consumption of 

 the mercury takes place. The mercury is volatilised ; but, on 

 the current being switched off and the lamp stopped, it is at 

 once recondensed and consequently no loss of mercury occurs. 

 The lamps usually have a very long life, excluding the risk of 

 accidental breakage of the tube, and their efficiency goes down 

 but very little even after running for a thousand or more hours. 

 This is a characteristic by no means common in electric lamps 



