ZOOLOGY AND BOTANY, MICKOSCOPYj ETC. 439 



each half subjected to two reflections before reaching the observer's 

 eye. In a subsequent improvement, a pair of prisms were placed with 

 the lower edges in contact, and rays transmitted with two refractions 

 and one reflection, reaching the eyes in such manner that the optical 

 angle was less than 14°. In either case, therefore, to adapt the eyes 

 to this condition, the internal rectus muscles were relaxed, and a 

 slight change of adjustment in the instrument was necessary. Dr. 

 Eiddell describes the result as follows : — ' Thus, a mite of a wheel- 

 animalcule, the 100th of an inch long, will perhaps appear to be a 

 foot off, and as large as a mouse ; but bring the prisms nearer together, 

 and tilt the oculars to correspond, and the image waxes marvellously 

 immense ; and, taking a position perhaps apparently more than 

 100 feet distant, the being, too small to be seen with the naked eye, 

 vies with the great whale of the ocean in size ; wearing an aspect 

 more awful to behold than the savage beasts of the African forests ; 

 exhibiting a complex transparent structure, more unique and wonderful 

 than the mind of man can well conceive.' 



We can good-naturedly forgive a little exuberance of imagination 

 when the reality which it accompanies is the first revelation from such 

 an instrument as that introduced to science by Dr. Eiddell." 



Visibility of Ruled Lines.* — Prof. W. A. Eogers states that he 

 has ruled bands of lines in which the lines were so fine and delicate 

 that they could not be seen with a Microscope, although their spacing 

 was much within the power of the Microscope to resolve. Yet he 

 was assured of the existence of the lines. The evidence in support of 

 this assertion was of three kinds : the pressure of the diamond upon 

 the glass was sufficient to produce a cut; the diamond produced a 

 peculiar singing sound while moving over the surface, which is always 

 indicative that it is working well ; and finally, the lines become 

 visible when filled with fine graphite. 



There is a limit beyond which lines cannot be satisfactorily filled 

 with graphite. It is difficult to fill lines finer than about l-80000th or 

 l-90000th of an inch. 



A most surprising result of some of the experiments of Prof. 

 Eogers, is that the naked eye can discern not only single lines that 

 cannot be seen with a Microscope, but that it can detect errors which 

 the Microscope will not show. Thus, he has a bar upon which lines 

 are distinctly visible to the unaided eye, and, although an objective 

 of low power will show them, one of high power will not. But even 

 errors or imperfections in ruling which cannot be seen or measured 

 with the Microscope, may reveal themselves to the eye by a peculiar 

 wavyness of the image. He attributes the failure of the objective to 

 show the lines, as mentioned above, to the inability to illuminate the 

 lines with light of the exact angle of incidence required, and the 

 proper angle of illumination he thinks deserves more careful 

 attention. 



* Amer. Mon. Micr. Journ., iv. (1883) pp. 45-6. 



