66 



SCIENCE PROGRESS 



In fig. 4 the incident rays are reflected upwards into the 

 condenser placed below the stage of the microscope ; they are 

 brought to a focus in the preparation and then enter the 

 objective. A portion of the back of the first lens of this objective 

 is made plane and blackened. This is chosen so that the whole 

 of the illuminating beam is absorbed, and therefore the only 

 light passing through the objective and reaching the observer's 

 eye will be light which is diffracted in the preparation. From 



i ■ 



Fig. 4B. 



the figure it can be seen that only a portion of the diffracted 

 rays is used, since some are absorbed by the blackened surface 

 of the lens. In Zeiss's objectives, all rays having an aperture 

 larger than 0*2 are available for observation, so that the loss 

 of light is small. It is obvious that it is immaterial whether 

 the objective be a dry or immersion system (fig. 4A). 



For observation with low magnifying powers, where the 

 aperture is usually smaller, this method becomes impracticable. 

 The arrangement of M. Zeiss and Co. shown in fig. 4B is more 

 convenient. The illuminating system has the larger aperture, 



