DIFFEACTIOX-BAND. 165 



facts and false doctrines as possible. It is due to other truth-seekers 

 that they should not be misled, to the great waste of their time and 

 pains, by our errors. And it is due to ourselves that we should not 

 commit our reputation to the chance of impairment by the prema- 

 ture formation and publication of conclusions, which may be at once 

 reversed by other observers better informed than ourselves, or may 

 be proved to be fallacious at some future time, perhaps even by our 

 own more extended and careful researches. The suspension of the 

 judgment, whenever there seems room for clouht, is a lesson incul- 

 cated by all those philosophers who have gained the highest repute 

 for practical wisdom ; and it is one which the Microscopist cannot 

 too soon learn, or too constantly practise. 



126. Besides these general warnings, however, certain special 

 cautions should be given to the young Microscopist, with regard to 

 errors into which he is liable to be led by the misinterpretation of 

 appearances peculiar to objects thus viewed, even when the very 

 best instruments are employed. Thus, the sharpness of the outline 

 of any transparent object is impaired by a change in the course of 

 the rays that merely £>ass by it, which is termed Injection or 

 Diffraction. If any Opaque object be held in the course of a cone 

 of rays diverging from a focus, the shadow which it will form upon 

 a screen held to receive it will not possess a well-defined edge, but 

 will have as its boundary a shaded band, gradually increasing in 

 brightness from the part of the screen on which the shadow is most 

 intense to that on which the illumination is most complete. If 

 the light be homogeneous in its quality, the shaded band will possess 

 no colours of its own ; but if the light be decomposable, like the 

 ordinary solar beam, the band will exhibit prismatic fringes.* It 

 is obvious that such a diffraction must exist in the rays transmitted 

 through the substance, as well as along the edges, of transparent 

 objects ; and that it must interfere with the perfect distinctness, 

 not merely of their outlines, but of their images, the various 

 markings of which are shadows of portions that afford obstacles, 

 more or less complete, to the perfectly free transmission of the 

 rays. There are many objects of great delicacy, in which the 

 ' diffraction-band ' is liable to be mistaken for the indication of an 

 actual substance ; on the other hand, the presence of an actual 

 substance of extreme transparence may sometimes be doubted or 

 denied, through its being erroneously attributed to the ' diffraction - 

 band.' No rules can be given for the avoidance of such errors, 

 since they can only be escaped by the discriminative power which 

 education and habit confer. The practised Microscopist, indeed, 



* This phenomenon is explained, on the Undulatory Theory of light, 

 by the disturbance which takes place in the onward propagation of 

 waves, when subsidiary centres of undulation are developed by the 

 impact of the principal undulations on obstacles in their course ; the 

 chromatic dispersion being due to the inequality in the lengths of the 

 undulations proper to the severally-coloured rays. 



