ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 147 



persons — a subject upon which we have found most observers are 

 particularly touchy. . . . Almost every one has eyes which are 

 more or less astigmatic ; how very different, therefore, must a set of 

 lines in the Microscope appear to most persons, according to the 

 relation of their direction to that of the meridian of astigmatism ! 

 Again, as to actual acuteness of vision — that is to say, power of visual 

 perception of minute objects — that varies in individuals to a degree 

 which could scarcely be believed by those not conversant with the 

 subject. . . . 



" The moral we would draw from these facts is that, when questions 

 of the eye against photography in the Microscope are discussed, the 

 conclusions arrived at are worthless unless the powers of the observer's 

 eyes are thoroughly ascertained, both as to acuteness of vision and 

 extent of astigmatism." 



The writer has not quite appreciated that the discussion turned, 

 not on the limit of visibility of minute objects, but on resolution or 

 the limit of visible separation. The latter depends on the wave- 

 lengths of the portion of the spectrum used, and hence photography 

 will resolve lines closer together than " white light " will. 



Imperfection of the Eye and Test Objects.* — Mr. L. Howe calls 



attention to the fact that " fine parallel lines, whether drawn artificially 

 or existing in natural objects, do not make fair test objects for the 

 Microscope." This is caused by an ocular imperfection which is 

 very common — astigmatism. 



In consequence of this defect, when one of Nobert's test-plates is 

 subjected to examination, the perpendicular lines which one person 

 can see perfectly well, cannot be seen by another who considers 

 his vision in every way normal. The same holds for other tests of a 

 similar nature, such as diatoms or objects marked with fine dots or 

 lines in close juxtaposition. This, the author says, is by no means 

 an imaginary difficulty, as it has occurred to him more than once to 

 find this difference of opinion between persons who are accustomed 

 to view such objects, and whose eyes and hands are trained to use the 

 Microscope. Fortunately, however, there is a very simple method of 

 overcoming the difficulty. This consists in revolving the object on 

 the stage of the Microscope, in such a way that lines which at first 

 were vertical become afterwards horizontal ; for when turned through 

 an arc of 180° they pass through every meridian in which it would 

 be possible to see them, provided the amplification and definition be 

 sufficient to make them at all visible. 



Pygidium of the Flea as a Test Object.f — Mr. E, M. Nelson 

 finds that the so-called hairs of the pygidium of the flea are spines 

 which " form nothing that can be called any sort of test for a high- 

 power objective." 



Webb's Lord's Prayer. — Mr. W. Webb, well known for his 

 microscopic writing of the Lord's Prayer (the series of which com- 



* The Microscope, v. (1885) pp. 226-8. 



t Journ. Quek. Micr. Club, ii. (1885) p. 197. 



L 2 



