Principles of Microscopy 19 



LIGHT SOURCES 



The light source, whether built in or not, is actually an integral part 

 of the microscope system. It is impossible to get the best service from 

 a first-class microscope with a second-class illuminator. A first-class illu- 

 minator improves a second-class microscope almost beyond belief. It is, 

 therefore, pathetic that people who will cheerfully spend the price of 

 an expensive automobile on a microscope will object to buying a lamp 

 costing less than a bicycle. 



A student microscope, with an illuminating system of the type shown in 

 Figs. 9 and 18, can operate satisfactorily with the light from a gooseneck 

 lamp fitted with a frosted or opal bulb. The large area of the illuminating 

 source and the low power of the condensing mirror combine to produce 

 adequate coverage for the field of a X3.5 (32-mm) objective. The stand- 

 ard X40 (4-mm) N.A. 0.65 fitted to these instruments will function with 

 this setup at an N.A. of about 0.30, which is just adequate for the crudest 

 type of biological observation. Numerous "illuminators," consisting of a 

 sheet of ground glass in front of a clear bulb, are on the market, but 

 they offer no advantage over an opal bulb. Some of these devices are 

 mounted on fancy adjustable stands and incorporate, for reasons apparent 

 only to the manufacturer, an iris in front of the ground glass. 



None of these devices is of very much use with a microscope incorporat- 

 ing a substage condenser since they do not permit the mirror ( Figs. 9, 10, 

 and 11) to reflect a beam of light large enough to fill the back lens of the 

 condenser. This has exactly the same effect as closing the iris (Fig. 13) 

 and thus removes the whole reason for using a substage condenser. 



The minimum requirements for a microscope illuminator for use with 

 a microscope fitted with a substage condenser are: 



1. A high-intensity lamp with a compact filament. This may be either 

 a 110-volt "projection" bulb or a 6-volt bulb worked through a transformer. 



2. A lens at least 3 in. in diameter, mounted in a focusing device, 

 which will produce a sharp image of the filament at a distance of from 

 6 to 12 in. 



3. An iris diaphragm in front of the lens. 



4. A filter holder in front of the iris. 



5. A housing that can be tilted. 



Most manufacturers can provide a lamp to these specifications, and Fig. 

 25 is typical of the best of them. It must be emphasized that such an 

 illuminator, the provision of which adds 25 per cent to the cost of a 

 "medical school" microscope, and 10 per cent to the cost of a research 

 microscope, is absolutely essential. There is no point in buying a X90 



