The Microscope. 227 



If, for example, a circular pin hole be made in a card, and 

 held at varying distances from the eye, it will appear horizon- 

 tally oval at one point and vertically or obliquely oval at an- 

 other. The same principle can be illustrated in still another 

 way. If upon a sheet of paper a number of equally dark lines 

 be drawn, radiating from a single point, like the spokes of a 

 wheel, and this figure be held a distance from the eye, it will be 

 found that certain ones of these lines appear to be darker than 

 others, although in reality they are all the same. It is unneces- 

 sary to multiply the experiments. The existence of this pecu- 

 liarity has long been well known to oculists under the name of 

 astigmatism. The great authority in the refraction and accom- 

 modation of the eye, Professor Donders, treats astigmatism as 

 one of the qualities of the normal eye. The practical fact is, 

 that we all possess this imperfection in a greater or less degree. 

 When it passes beyond a certain point it then is classed as mor- 

 bid, and requires suitable glasses to correct it. Now the point 

 to which I wish to direct your attention is, that when a person 

 looks through the microscope at a series of fine parallel lines it 

 occurs very frequently that they can not be seen on account of 

 this peculiarity of his eye. At any rate when one of Noberts' 

 test plates is subjected to examination the perpendicular lines 

 which one person would see perfectly well would not be seen 

 by another person who might consider his vision in every way 

 normal. The same, of course, holds for other tests of a similar 

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

 in close juxtaposition. This is by no means an imaginary diffi- 

 culty, as it has occurred to me more than once to find this dif- 

 ference of opinion between persons who are accustomed to view 

 suoh 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 one hundred and eighty de- 

 grees 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. This question or the re- 

 lation of astigmatism to test objects for the microscope, seems to 



