286 Transactions of the Society. 



aperture of which is not less than three-quarters of the N.A. of 

 the observing objective (B). 



To armnge a Microscope for Critical Illumination. — 1. Place a 

 low power, say a 1-in., §-in. or ^-in., upon the nose-piece, an object 

 upon the stage, and a condenser in the substage. Incline the 

 Microscope and point it directly at the source of light, which should 

 be the edge of the flame from a ^-in. wick of a paraffin lamp. 

 Fill the field with any kind of light by racking down the condenser, 

 and bring the object, or a well-marked part of the object, to the 

 centre of the field. 



2. Close the iris -diaphragm, or if there is not one place a 

 diaphragm with a small hole in it beneath the substage condenser, 

 and rack the condenser so that the image of this hole is focused 

 upon the plane of the object on the stage, and by means of the 

 substage centring screws bring the image of this hole central with 

 the centring object. 



3. Eack up the condenser further until the source of light comes 

 into focus in the plane of the centring object. If the source of 

 light does not happen to be central with the centring object, the 

 lamp must be moved so that the image of the source of light shall 

 be superimposed on the centring object. (On no account must the 

 centring screws of the substage condenser be used for this purpose, 

 only the lamp must be moved.) 



4. Remove the low power used for centring and place the 

 object glass which is going to be used on the nose-piece. If the 

 object and the image of the source of light are not central to 

 the new field, first place the centring object central to the new 

 field, and then by means of the substage centring sere svs bring the 

 image of the source of light central to it. (On no account must the 

 source of light be moved for this purpose.) 



5. The eye-piece is now removed, and the back lens of the 

 objective is examined by looking down the Microscope body. 

 Open the iris, or other diaphragm, until it is seen that three-quarters 

 of the back lens of the object glass is full of light, as in B. 

 Note. — A disk of light as in B cannot be obtained unless the 

 aperture of the substage condenser is aplanatic to that extent. 



6. It is usual to go over Nos. 2 and 3 a second time ; thus, when 

 in (3) the lamp has been moved so that the image of the edge of 

 the flame is central with the centring object, rack down the sub- 

 stage condenser until the diaphragm hole comes into focus, and 

 note if it is still central to the centring object, if not re-centre it 

 by means of the substage centring screws. Next rack up the 

 substage condenser until the image of the edge of the flame is in 

 focus, and if necessary re-centre it to the centring object by moving 

 the lamp. 



If it is necessary to use a Microscope in a vertical position, the 

 above procedure should be gone through with the image of the 



