20 The Microscope 



N.A. 1.2 oil-immersion objective and an N.A. 0.9 or higher substage con- 

 denser if the system cannot be used at better than N.A. 0.6. Just as useful, 

 that is, as large and as well-resolved, an image can be obtained with a 

 X45 N.A. 0.6 objective and a X20 eyepiece at less than half the price. 



The method of using the illuminator is described in the next chapter 

 but the reasons for its parts can be given here. An ideal image is pro- 

 duced when all those rays that leave the light source at one time also 

 reach the eye at one time. It follows that an ideal light source would 

 be a point. A compact filament is a compromise between theory and 

 practicality. The lens is required to gather the rays leaving the filament 

 and condense them into a beam of useful size and direction. This lens, 

 as will be seen later, is in practice the light source of which the substage 

 condenser forms an image on the slide. The iris in front of it is there- 

 fore required to limit the size of this image. This brings up the very 

 important point of how to control the intensity of illumination, since 

 neither the substage iris nor the lamp iris can be used for this purpose. 

 There are two methods. The first is to vary the voltage applied to the 

 lamp. This is simple but has the disadvantage that the color of the 

 light becomes redder as the intensity is diminished. Much the best 

 method is to place neutral density filters in the filter holder in front of the 

 iris. 



The high cost of providing the necessary illuminator, and the difficulty 

 of persuading the customer of the necessity for it, is causing more and 

 more manufacturers to turn to built-in illuminators. Those attached to the 

 underside of the substage condenser are not very satisfactory, both be- 

 cause of the heat generated and because the cramped situation makes it 

 difficult to provide an adequate condensing system. Those instruments, 

 however, that have the illuminating system built into the stand are in 

 general excellent. 



The most satisfactory microscope illuminant on the market at present 

 is the American Optical (formerly Silge and Kuhne) Ortho-Illuminator 

 (Fig. 35). This relies on the new idea of using a series of fluorescent glass 

 discs as the source of illumination. Three such discs, mounted on a 

 swiveled plate, provide three practical light intensities of unvarying color. 

 Red, green, and "daylight" filters, also on a swiveled plate, may be 

 rotated in front of any of them. Both fluorescent disc and filter may be 

 rotated out of the way to permit direct centering of the lamp filament. 

 Once a microscope has been set up on this device, it is only necessary 

 to switch on the lamp to have three possible intensities of three colors 

 perfectly adjusted at all times. 



