WORKING DISTANCE 65 



focus all over by mounting it in Canada balsam, or by 

 immersing it in water. The latter is a device sometimes 

 useful when photographing fairly large objects. 



Flatness of Field. A " flat field " is one in which 

 objects at the centre and periphery are simultaneously 

 in focus, and it is very desirable that low-power objectives 

 should simultaneously focus every part of an object that 

 they cover. At present it is not possible to combine this 

 property with perfect corrections in an objective of the 

 ordinary construction, and when a photomicrograph of 

 a large surface is required it is best to use one of the small 

 photographic lenses specially constructed for that pur- 

 pose, and described on page 79. High-power objectives 

 are usually required to give an image of a small part of 

 the field with perfect detail, and great flatness of field 

 cannot be expected under these circumstances from an 

 objective of higher power than J" or J". The roundness 

 of field of high-power apochromats is very noticeable, 

 but the real test of a high-power objective is found in the 

 perfection of definition of an object focussed at the edge 

 of the field, which should be as good as when focussed at 

 the centre. 



Working Distance is the distance between the front 

 lens of the objective and the object, or cover-glass, when in 

 focus. It must not be confused with focal length, which 

 is not measured from the front lens, but from the lower 

 equivalent plane of the objective, which may be above 

 or below the front lens, according to the construction 

 of the combination. Lenses of the same power vary 

 considerably in working distance, which becomes im- 

 portant in those of J" and upwards, particularly when 

 thick mounts have to be examined. It is also much 

 easier to focus rapidly a lens of long-working distance 

 than one which involves very near approach to the 

 object, and consequent danger of fracture of the cover- 

 glass. 



