64 VISION WITH THE COMPOUND MICEOSCOPE 



of microscopic vision given up to this time, it has been usual to 

 state that he held and taught that the microscopic image consists of 

 two superimposed images, each having a distinct character as well 

 as a different origin, and capable of being separated and examined 

 apart from each other. The one called the ' absorption image ' is a 

 similitude of the object itself, an image of the main outlines of the 

 larger parts ; but by the other image all minute structures, striation, 

 and delicate complexity of detail whose elements lie so close together 

 as to occasion diffraction phenomena can alone be formed, because 

 these could not be geometrically imaged. So that in the case of an 

 object with lines closer than the ^sVo ^ an i ncn apart, the image 

 seen by the eye is formed, not simply by the central dioptric beam, 

 but by the joint action of that and the superimposed diffraction 

 images, and their exact union in the upper focal plane of the objective. 

 The first of these was held to be a negative image, representing 

 geometrically the constituent parts of the object ; but the second 

 was considered a positive image because it delineates structure, the 

 parts of which appear self-luminous on account of the diffraction 

 phenomena which they cause. It was this ' diffraction image ' that 

 was said to be the instrument of what has so long been known as 

 the * resolving ' power of lenses. 



But Dr. Abbe, with the full light of further investigation and 

 experience, does not hesitate to modify this explanation. He says : 

 ' I no longer maintain in principle the distinction between the 

 " absorption image " (or direct dioptrical image) and the " diffraction 

 image," nor do I hold that the microscopical image of an object 

 consists of two superimposed images of different origin or different 

 mode of production. 



* This distinction, which, in fact, I made in my first paper of 1873, 

 arose from the limited experimental character of my first researches 

 and the want of a more exhaustive theoretical consideration at that 

 period. I was not then able to observe in the microscope the dif- 

 fraction effect produced by relatively coarse objects because my 

 experiments were not made with objectives of sufficiently long focus ; 

 hence it appeared that coarse objects (or the outlines of objects 

 containing fine structural details) were depicted by the directly 

 transmitted beam of light solely, without the co-operation of diffracted 

 light. 



' My views on this subject have undergone important modifica- 

 tions. Theoretical considerations have led me to the conclusion 

 that there must always be the same conditions of the delineation as 

 long as the objects are depicted by means of transmitted or reflected 

 light, whether the objects are of coarse or very fine structure. 

 Further experiments with a large microscope, having an objective 

 of about twelve inches focal length, have enabled me to actually 

 observe the diffraction effect and its influence on the image, viewing 

 gratings of not more than forty lines per inch. 1 



1 Diffraction effects may be observed without a microscope ; they can be easily 

 demonstrated by observing a lamp- flame through a linen pocket-handkerchief or a 

 fine gauze wire blind. This can be done readily by placing the eye close to the linen 

 or wire. 



