66 DISCOVERY CH. 



to be ruled within the width of an inch. All the parts 

 of the machine by which a surface as hard as steel has 

 thus traced upon it, by means of a diamond, a furrow 

 several miles long, have to be made with extreme 

 accuracy. A screw to rule a " grating " six inches 

 wide required two years or more to make ; and almost 

 ten years were occupied in making one to rule a " grat- 

 ing " fifteen inches wide. From beginning to end the 

 process of producing a perfect optical grating is beset 

 with difficulties, and only an indomitable spirit can 

 overcome them. Nothing must be left to chance ; 

 and the slightest disregard of a possible disturbing 

 influence during the grinding of the screw or the ruling 

 of the grating means that months or years of work will be 

 completely wasted. 



Prof. A. A. Michelson, who devoted many years to 

 the perfection of a machine for producing accurate 

 optical gratings, has compared it to an antagonist who 

 was always ready to take advantage of a mistake, yet 

 always played fair. 



One comes to regard the machine as having a personality 

 I had almost said a feminine personality requiring humouring, 

 coaxing, cajoling, even threatening ! But finally one realises 

 that the personality is that of an alert and skilful player in an 

 intricate but fascinating game who will take immediate advantage 

 of the mistakes of his opponent, who " springs " the most dis- 

 concerting surprises, who never leaves any result to chance, but 

 who nevertheless plays fair, in strict accordance with the rules 

 of the game. These rules he knows, and makes no allowance if 

 you do not. When you learn them, and play accordingly, the 

 game progresses as it should. Prof. A. A. Michelson. 



In order to test certain conclusions he had reached 

 as to the effect of the earth's atmosphere in absorbing 

 particular rays of sunlight, a French astronomer, Dr. 

 Jules Janssen, wished to make observations at the 



