A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 205 



far as it goes, be a true rendering of existent structure. The 

 diffraction theory, while teaching that the central narrow incident 

 pencil can alone be expected to yield strictly similar images, holds 

 out but poor assurances of their reliability to the investigator, for 

 we are told that " minute structural details are not imaged by 

 the microscope geometrically oi- dioptrically, and cannot be inter- 

 preted as images of material forms, but only as signs of material 

 differences of composition of the particles composing the object, 

 so that nothing more can be safely inferred from the image as 

 presented to the eye than the presence in the object of such 

 structural peculiarities as will produce the specific diffraction 

 phenomena on which the images depend." * Judging from 

 practical observation, many of us will cordially endorse the 

 truth of this inference when non-critical, narrow-cone images 

 are concerned. Every optically careless rough-and-ready micro- 

 scopist would therefoie do well to bear in mind the danger he 

 incurs should he even inadvertently employ such methods, as in 

 consequence he will probably eventually discover that the con- 

 clusions he has reached after years of patient microscopical 

 research are untrustworthy, having been vitiated through the 

 lack of a little manipulative skill, which might have been quickly 

 and easily acquired ; for with small -cone diffraction images no 

 ordinary non-mathematical observer is fairly entitled to hold or 

 express an opinion regarding the interpretation of what he sees, 

 because, in order to understand its real significance, he must 

 remove the eyepiece and examine the diffraction spectra at the 

 back of the objective, and from these he must mathematically 

 compute the structure by which they have been produced, 

 remembering that the ideal picture resultant from his calcu- 

 lations will only prove a completely true representation of the 

 object in the event of all its diffraction spectra having been 

 admitted by the objective. Only imagiae an animated discussion 

 on some keenly- controverted point in the internal organisation of 

 a minute saprophyte or infusoiian conducted on such a basis ! 

 Even where simple diatomic structures are concerned, no more 

 impressive demonstration of the hopelessness of correct compu- 

 tations of the kind is to be found than in the well-known 

 examples of Dr. Eichhorn's calculation of the structure. 



* Carpenter's " The Microscope," 8th edition, p. 72, 

 JouRN. Q. M. C, Series II.— No. 50. 14 



