1881.] 



MICEOSCOPICAL JOURNAL. 



91 



capabilities of every compound micro- 

 scope, that can be definitely expressed 

 in figures, and that this limit primarily 

 depends upon the angular-aperture 

 of the objective. 



I will now call your attention to 

 the fact that the images of minute 

 objects which the microscope gives 

 are no proof whatever of the real 

 constitution of the object. Many 

 years ago it was observed that the 

 microscopical appearance of an ob- 

 ject could be greatly changed by al- 

 tering the conditions of illumination, 

 and this fact gave rise to much dis- 

 cussion about the structure of cer- 

 tain insect-scales and the nature of 

 the fine markings of diatoms. On 

 one side it was held, that be- 

 cause changes in the illumination 

 and delicate alterations of the focus 

 produced such positive differences 

 in the image, it was impossible to 

 demonstrate the structure of such 

 objects by means of the microscope. 

 For example, there was no means of 

 determining whether the dots upon 

 the diatom-frustules, or the markings 

 on the podura-scale, were elevations 

 or depressions, for they could be 

 made to appear either light or dark 

 by turning the focussing screw. On 

 the other side it was maintained that 

 the perfection of the image in the 

 microscope, when the best objectives 

 were used, was ample proof that the 

 structure was correctly delineated ; 

 and this, 1 believe, is the prevailing 

 impression at the present time. 

 However, we have already learned 

 that the diffractive action of the ob- 

 ject determines the character of the 

 image, and we must therefore con- 

 clude that the' apparent excellence of 

 this image is no criterion from which 

 we may infer the real structure of 

 the object. As a matter of fact we 

 can prove, by direct experiment, that 

 this is true. For example, we may 

 rule a series of lines upon glass in 

 such a manner that the diffractive 

 effect will be identical with that of 

 P. angulatum. The image of these 

 lines formed by the microscope will 



be undistinguishable from that of the 

 diatom ; and the same will be true of 

 any object which gives the same dif- 

 fraction spectrum, whether it be a 

 series of ruhd lines or of minute 

 particles, or any structure whatever. 



When F. Angulatum is examined 

 with a certain kind of illumination, 

 we will see a series of lines running 

 directly across the frustule ; by 

 changing the direction of the illu- 

 mination the lines will appear to be 

 inclined to their original direction, 

 and by still further manipulation the 

 lines can be made to disappear, and 

 sharply-defined dots will take their 

 place. When the focussing-screw is 

 changed, the dots will change from 

 dark to light, or the reverse. All these 

 appearances can be perfectly repro- 

 duced by substituting a glass slip, with 

 two series of lines crossing each other 

 at an angle of 6o". It is by the use 

 of artificial test-objects, such as rul- 

 ings on glass, the exact nature of which 

 is known, that we are able to verify the 

 deductions from theory. Perhaps 

 the most striking evidence that we 

 have of the unconformability of the 

 image with the structure of the ob- 

 ject, is to be found in the fact that 

 when we examine a ruled plate, having 

 a known number of lines in a given 

 space, it is possible to cut out some 

 of the diffraction spectra, so that the 

 number of lines in that space will be 

 doubled in the image. In such an 

 experiment it is impossible to distin- 

 guish the false lines from the true ; 

 for both depend upon the physical 

 action of the object upon the light, 

 and neither image is a real image, 

 in the ordinary acceptation of the 

 term. 



In conclusion I desire to say a few 

 words about the bearing of the facts 

 which have been brought forward. 

 It is discouraging to the scientific in- 

 vestigator, who uses the microscope, 

 to learn that no skill, no care or 

 judgement will enable him to always 

 distinguish the true from the false. 

 Nevertheless, this is precisely the posi- 

 tion of the microscopist, whenever he 



