The Microscope. 225 



adopted. A few lines are ruled, examined under the microscope, 

 and if their character is unsatisfactory, the inclination of the 

 diamond to the plate is changed, more lines are ruled, and ex- 

 amined, till a suitable crystal is obtained. Prof. Rogers informs 

 me that he has never yet been able to see a ruling crystal under 

 the microscope, and only knows their existence from their 

 effects. The time consumed in finding a ruling crystal is often 

 considerable. I should say that at least 3 hours were spent in 

 search before one was found that was satisfactory for the work 

 done by Prof. R. in my presence upon the occasion referred to 

 at the beginning of this article ; and I suppose that it would 

 have taken one of less experience much longer. 



A satisfactory ruling crystal having been found, if the bar 

 to be ruled is of such a length, say one decimeter, that a 

 correction for temperature is necessary, a trial set of lines is 

 ruled at any convenient distance, say one centimeter, apart. 

 The plate is then put upon the bed of a comparator, and allowed 

 to remain there long enough to acquire the temperature of the 

 comparator, and a comparison made with a standard bar of the 

 same material as the bar to be ruled. The difference in length, 

 if any, having been ascertained, the correction necessaiy to make 

 the plate standard at 62° F. is applied, and the ruling done in the 

 manner above described, the finer rulings being made automat- 

 ically, using a part of the precision screw for which the correc- 

 tions are the most nearly uniform, and the larger spaces being 

 ruled without the use of the automatic part of the machine. 



During the time the automatic ruling is being done, the 

 operator occasionally looks through the microscope at the revolv- 

 ing index to see that no farther correction is necessary. If at any 

 time the accumulated error appears from the index to equal two 

 divisions of the eye-piece micrometer, (1-100,000 inch) correc- 

 tion is at once applied ; but the machine is so accurate, that 

 when the setting of the index, regulating the distance between 

 the stops, is once correctly made, such further correction is not 

 often necessary. 



AVhen a set of graduations has been ruled, if it is desirable 

 to rule another set on the same plate, the precision screw is 

 turned back to zero of position, the length of the rod moving 

 the diamond and holder changed, and the new set ruled ; and so 



