60 SCIENCE PROGRESS 



moved from neighbouring objects. In an image of this 

 formed by means of a lens system, there will be a small 

 patch of diffracted light of sensible dimensions.; and if the 

 background be sufficiently dark compared with the intensity 

 of this patch, the latter will show up. This will happen 

 whatever the size of the particle, providing the intensity of 

 the patch of diffracted light is great enough. 



An analogy will make this clearer. As in the case of the 

 microscope, so in that of the telescope, there is a limit to the 

 resolving power. Here, of course, objects at different distances 

 are observed ; the resolving power depends on the angular 

 distance of the object. If two points subtend less than a 

 certain angle at the telescope, their images can no longer be 

 separated ; or if an object subtends too small an angle, 

 though it may be visible, it cannot be studied. Now it is 

 well known that there are some stars the forms and details 

 of which cannot be observed even with very large telescopes, 

 yet they may be visible to the naked eye. All that is known 

 concerning them is their colour and brightness, which serve 

 as a basis of classification. 



Just as stars are only visible to the naked eye at night, so 

 the small luminous particles considered above will be seen to 

 the best advantage against a dark background ; and the 

 smaller the object, the more brightly illuminated must it be 

 to be visible. Moreover, in the telescope there is more light 

 concentrated into any image than there is with the naked 

 eye, and so a greater number of stars is observed through 

 the telescope than without it. In the same way, the micro- 

 scope by concentrating the light will render visible a greater 

 number of ultramicroscopic particles ; for the greater concen- 

 tration of the light will reveal some particles which could 

 not be seen with the naked eye. Again, the smallest angular 

 distance of two objects which can be separated by the eye 

 is about one minute, and this will set a limit to the distance 

 between two ultramicroscopic particles which can be seen 

 separately. It can be readily understood that with the 

 assistance of the microscope this minimum distance will be 

 diminished, but even with the microscope there will be 

 clusters of particles which cannot be resolved. It is obvious 

 then that in the observation of ultramicroscopic particles the 

 microscope will be of great assistance, not only because of 



