14 



PROGRESS IN MICROSCOPY 



mentioned aluminium-based coating and a high-magnification eyepiece. 

 After selecting a hole small enough to exhibit the conventional features 

 of Airy's disk, it is brought in the centre of the field and the fine 

 adjustment actuated. Then, the microscope is brought nearer the 

 specimen whereupon the structure of the diffraction-disk is materially 

 changed. Now, reverting to proper focusing, the operation is resumed 

 by moving the objective away from the specimen. If the objective is 

 well corrected, the same successive aspects as before are retained. 

 If, conversely, the objective is not well corrected the aspect of 

 the diffraction patterns is very different. On one side many diffraction 

 rings are seen whereas, on the other, there are more or less uniform 

 blobs. Strange as it may seem, such examination is quite easy to 

 carry out, even by a layman, because the eye is very sensitive to small 

 differences in appearance provided that the to-and-fro motion imparted 

 to the focusing device be not too slow. 



5. IMAGE OF A LUMINOUS POINT IN WHITE LIGHT 



Owing to chromatism of the objective, im.ages correlated to the 

 various wave-lengths of the spectrum are not focused simultaneously. 

 Assuming that the image A'^^, correlated to wave-length /. =0-56/^ 

 in Fig. 1.15. is being focused: at wave-length / = 065 /<. for instance. 



Fig. 1.15. Depth of fojus v produced by chromatism. 



the image would be at 5^ and the depth of focus equal to .v. Image 

 structures originated by the various radiations can be theoretically 

 computed. Intensity at any point of the focusing plane .t equates the 

 sum, at the said point, of the total illumination generated by the 

 various radiations. It follows that the spectral distribution of energy 

 emitted by the source and the spectral responsiveness of the receiver, 

 (either the eye or photographic plate) are to be taken into account. 

 In perfectly chromatically-corrected microscope objectives as, e.g. in 

 reflecting objectives, all images correlated to the various wave-lenglhs 

 are Airy's disks in the plane -t. Formula 1.2 shows thai red images 



