OF ARTS AND SCIENCES. 3S 



may be as great as 100 feet. It is only necessary that the plane face 

 of the prism shall be directed towards the source of light. In order 

 to get the best results, the power of the objective should be as great as 

 the equivalent of au inch lens. But if an objective of great working 

 distance is required, a large prism may be mounted in front of the lens. 

 The writer has made use of this method of illumination for the micro- 

 scopes of the Harvard College Meridian Circle with good success. 

 The focal length of these objectives is four inches. 



The following are the results of the various determinations of the 

 values of the micrometer screws. 



Comparator No. 1, Microscope A, UniTersal Comparator, Microscope A, 



1 inch objective. 1 inch objective. 



0.441 

 0.441 

 0.442 

 0.440 



0.440 



It will not escape attention that no mention has thus far been made 

 of any arrangement for protecting the standard bars to be compared 

 from the effect of the increase of temperature due to the presence of 

 the observer in the comparing-room. The omission has not been 

 accidental. 



The common impression, that the effect of such increase of temper- 

 ature will be immediately apparent, is erroneous. If the meter is 

 traced upon a thin ribbon of steel, any i^jcrease of temperature amount- 

 ing to 10° will produce the change in length which its coefficient of 

 expansion demands, within 15 or 20 seconds ; but if its mass be in- 

 creased two hundred fold, a change in temperature of the same amount 

 will require from one to two hours for its normal action. 



The preparatory work which needs to be done with evei-y bar which 

 is to receive standard graduations is the determination of the time 

 required for a given change of temperature to produce its full effect. 

 This time is a function of the shape and the mass of the bar. It is 

 proposed during the approaching winter to make an exhaustive study 

 of this element for the bars under consideration. The few observa- 

 tions which have already been made seem to show that, if these bars 

 are quickly removed from a temperature of 32° to a constant temper- 



