ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 137 



matter to rule lines whicli are easily visible by the unaided eye, 

 especially in sunlight, having a width not exceeding 1/50,000 in., 

 I have never since succeeded in obtaining a plate quite as good as 

 the one described. Clearly the ruling crystal had been broken 

 off before this particular plate was ruled, and, as often happens, a 

 minute and delicate crystal remained, which produced the lines which 

 were really traced. 



In the course of subsequent experiments I found that while the 

 visibility was increased by the film of moisture, exceedingly fine 

 lines could be seen without this aid to vision when the proper angles 

 of inclination to the source of light are obtained. To get the best 

 results the ruled surface should have an angle of about 15° with 

 the source of light, and the lines themselves should have nearly the 

 same angle of inclination. Everything depends upon getting the exact 

 angles of inclination required. More striking results are obtained by 

 sunlight than by artificial light. Highly polished metals, especially 

 tempered steel and iridium, yield better results than glass. I will 

 not undertake to say how fine lines traced upon metal can be seen, 

 but I suspect that the limit of naked-eye visibility is far beyond the 

 capacity of ruling. I have a plate of highly polished and nearly pure 

 iridium upon which there are traced a series of lines which are dis- 

 cernible by the eye in sunlight, but which I have never yet been able 

 to see under the Microscope by direct light. Yet these lines are 

 easily seen with a low-power objective under certain conditions.! 



I do not propose to offer any theory to account for the facts which 

 I have observed, not even the one which would naturally be the one 

 first suggested — viz, that of visibility by reflection. I admit that the 

 apparent width of the lines would be increased if the real and reflected 

 lines could be seen side by side. It can be easily shown that the 

 lines in one of the accompanying plates are visible under conditions 

 in which it is impossible for reflection to take place. For the present 

 I content myself with stating the facts of observation illustrated by 

 the ruled plates by which these observations can be repeated. 



I close this paper with the suggestion that the increase in the 

 efficiency of the Microscope will probably come from the better 

 manipulation of the light under which an object is viewed. At present 

 the unaided eye is a not very unequal competitor of the Microscope 

 in the matter of simple vision. In fact, there are certain phenomena 

 connected with this question which can be better studied by the un- 

 aided eye than under the Microscope. I believe it to be possible to 

 see under the action of sunlight what cannot be seen under any 

 objective. There has been produced upon my ruling-machine, upon 

 a polished surface of tempered steel, a band of 10,000 lines, cover- 

 ing a space of 4 inches. I have tested the equality of the spacing 

 for aliquot parts of a revolution of the screw in every possible way 

 by direct measurement. Other observers have done the same thing. I 

 can hardly be wrong in the assertion that the spaces indicated by even 

 tenths of a revolution are exactly equal as far as any tests of direct 

 measurement can be applied. Yet, by holding this bar in a certain 

 position with respect to the source of light, the limits of each revolu- 



