28 



THE MICROSCOPE 



Lmnersion 

 ac^omatic 

 condenser. 



Use of a 

 Bubstage 

 condenser. 



To focus a 



substage 



condenser. 



<i*^ 



Fig. 19.— No. 3291, Dry and 

 Immersion Condenser. 



illiiroinated by a hollow cone. Stops or patch-stops with apertures 

 of difierent shapes, or in difierent positions below the condenser, 

 used in combination with the iris diaphragm, regulate the illumina- 

 tion so that light in any direction may be passed through the 

 object. 



The dry and immersion achromatic condenser is of even higher 

 quality than the achromatic condenser,being equal in its corrections 

 to a microscope object glass, and has an aperture of 1'3 N.A., so 



that it can, if used in immer- 

 sion contact with the under- 

 surface of the slide, fiU the whole 

 aperture of an oil-immersion lens 

 with light. 



For transparent objects with 

 object glasses as low in power as 

 a 1^-inch (32-mm.) a condenser 

 is not required. The aperture of 

 such a low-power lens is small, 

 and the angle obtained by the 

 use of a concave mirror is usually 

 sufficient to make the best use of 

 this lens. 

 The same applies to some extent to the 2/3-inch (16-mm.) 

 object glass, but as this lens is so frequently used as a finder for 

 a high-power, it is not always convenient to rapidly remove the 

 condenser, and it is customary to use the condenser, but to put 

 it somewhat out of focus in order to fill the whole field with an 

 even illumination. A condenser should always be used with 

 the 1/3-inch {8-mm.), 1/6-inch (4-mm.), 1/8-inch (3-mm.), or 

 1/12-inch (2-mm.) object glasses. 



A substage condenser is only corrected for light which is 

 parallel or slightly divergent ; therefore the flat mirror should be 

 used. The concave mirror giving convergent light is quite un- 

 suitable for use with a condenser. 



Daylight as a source of light is not recommended with a 

 condenser, as the finest detail cannot be shown by its means. 

 For ordinary microscopic examination of not too critical a 

 nature, daylight is satisfactory, but even then the more delicate 

 details may escape notice. 



Assuming that the source of light is a paraffin lamp with a 

 flat wick, or other small source of illumination, the condenser 

 must first be focussed and centred. 



In order to focus the condenser, sufficient light must be thrown 

 through the object to render it visible, and the object glass must 

 be focussed upon the object. The iris diaphragm of the condenser 

 should then be shut down to about one-quarter its size, and it 

 shoidd be focussed up and down until an image of the flame of 

 the lamp is seen sharply in focus at the same time as that of the 



