OF ARTS AND SCIENCES. 255 



the gradual outgrowth of experimeuts undertaken for a different pur- 

 pose. Indeed, whatever has been accomplished thu3 far may be said 

 to be the result of an unsuccessful search after a spi<ler that would 

 spin a web suitable for the meridian circle of Harvard College Obser- 

 vatory. Failing to find suitable " spider lines," an effort was made 

 to produce on glass, lines of the desired quality and size. This was 

 finally accomplished by etching with hydrofluoric fumes ; the lines 

 having been first ruled in a coating formed by dissolving white wax in 

 gasolene, and uniting the solution by emulsion with liquid gelatine. 

 The coating thus formed will receive lines as fine as 10,000 to the 

 inch, while its protecting qualities are sufficient to withstand very 

 strong fumes. The subsequent experiments detailed in this paper 

 have occupied my attention from time to time during the past three 

 years, when not engaged in my regular duties as Assistant iu the 

 Observatory. 



