Illumination and Illuminating Apparatus 97 



focused with the J-inch or other low-power objective, and 

 the sub-stage condenser focused upon the object. The high 

 power may then replace the low power, and the condenser 

 will be in adjustment. If it be found that the image of 

 the lamp-flame is not in the middle portion of the field on 

 exchanging the objectives, it will show that the objectives 

 have not exactly the same centres, and the image must 

 be set central with the high power by altering the position 

 of the condenser by means of the centring screws of the 

 sub-stage. 



The next question is, What amount of light should be 

 admitted from the condenser in order to see the object at 

 its best ? Mr. Nelson has suggested that the aperture of 

 the condenser should be about three-quarters that of the 

 objective, and in order to arrange this it will be necessary 

 to remove the eyepiece from the microscope, and look 

 down the tube at the back of the object glass, opening the 

 diaphragm of the condenser to its fullest extent. Bearing 

 in mind the size of the circle of light seen, gradually diminish 

 the opening of the diaphragm of the condenser until one 

 quarter of the back lens of the objective is shut out ; again 

 put in the eyepiece, and the desired amount of illumination 

 is arranged. The aperture employed should be varied 

 slightly according to the transparency or opacity of the 

 object under view. 



When the condenser is centred and focused, and the 

 back lens of the objective is three-quarters 

 filled with light, a critical image is obtained ; 

 that is, the objective is understood then to 

 produce the finest results it is capable of. 



Mr. Nelson's f-cone method of illumination 

 has been almost universally accepted as a FlG - 32. BACK 



most practical one ; but the following plan, ( 



which was suggested to the writer by a 

 microscopical friend, has given very satisfactory results, 

 On examining the back lens of the objective with a striated 

 object, such as Pleurosigma angulatum, resolved and focused 



7 



