222 The Microscope. 



Referring to the sentence that I did not seem to recognize the 

 lines nearly as far up in the series as 120,000 lines per inch: if this 

 were the case, how could I say that there are lines there ? When 

 ruling, I naturally have to know that I am cutting lines, and when I 

 rule 50,000 lines per inch, I have to see that there is room enough 

 for three more lines between a divison of that denomination; other- 

 wise it would be useless for me to assert that I have ruled 200,000 

 lines per inch. Besides, I did not consider myself one of the com- 

 mittee, and did not have anything to say. 



Answering the question that Dr. Ward put regarding the 

 retinal impressions : Each person, when observing the lines, did not 

 run over the whole 23 bands consecutively; but rests were made at 

 intervals for some minutes, and when the highest attainable band 

 was reached it was allowed to remain untouched for some time 

 again, so that the observer might convince himself if he did really 

 see what he claimed — seeing plainly lines and spaces — and that it 

 was no retinal impression of the lower bands. However, the eye 

 will not hold the image a great length of time. 



Further, Dr. Ward asserts that the lines should be counted and 

 measured, which would be very tedious and uncertain work without 

 a microscope and micrometer, as I have constructed them, viz: for 

 fine measurements and resolutions. Regarding which, the late Dr. 

 J. J. Woodward said that it worked " with great precision." And 

 were they compared with a micrometer one per cent, short, the 

 100,000 would be — according to that micrometer — only 99,000 lines 

 per inch. Does the Doctor wish to insinu.ate that I try to deceive 

 those interested therein by saying that there are 150,000 or 200,000 

 lines per inch when they are not ruled in so high a denomination ? 

 Dr. Van Allen counted the end lines of 190,000, which are virtually 

 95,000 lines per inch, and said that it was a severe strain on the 

 eyes. I place my microscope for the use of anybody who wishes 

 to count and measure the lines, and he may convince himself of the 

 correctness of the ruling by comparing it with the U. S. standard, 

 which, after a number of months of measurements was placed as 

 a standard by the officers of the U. S. Coast Survey, and Dr. J. J. 

 Woodward. 



We cannot measure the lines proper, but we can measure the 

 image, and here the illumination will have to be taken into consid- 

 eration, which was not done by the " Committee on Micrometry," of 

 which Dr. Ward and Prof. Rogers were members, when they 



