The Microscope. 47 



was the initial cause of a multitude of subsequent experiments and 

 observations, extending over a period of sixteen years, and which 

 resulted in earning the investigator an enviable reputation, and mak- 

 ing him a universally acknowledged authority in all that pei'tains to 

 micrometrical work. 



Being unsuccessful in the spider hunt, he began a series of 

 experiments for the pui'pose of finding a practical method of etching 

 lines on glass. After many fruitless efPorts, the desired result was 

 obtained by making a film on the glass; cutting the lines to be 

 etched through this film, by means of a steel point, and exposing the 

 plate thus prepared to the fumes of hydx'ofluoric acid confined 

 in a closed vessel. This gave a sharply defined, etched line. A 

 satisfactory production could only be obtained, however, when the 

 confined atmosphere had reached a certain degree of saturation with 

 the acid. An exposure of the glass in an atmosphere below or 

 above the the proper standard, yielded inferior or worthless lines. 

 So successful did this method become, in the hands of its originator, 

 that he furnished etched plates under an order from the U. S. 

 government, to be used in photographs taken by the expeditions 

 sent out from this country to observe the transits of Venus. 



About this time he also became interested in the construction of 

 comparators for the determination of difPerences in length, and 

 by their aid, establishing useful, working standards of measurements 

 for practical mechanical work. In connection with Geo. M. Bond, 

 of Hartford, Conn., Professor Rogers designed and supervised the 

 construction of what is known as the Rogers-Bond Universal Com- 

 parator. Two of these instruments were built by the Pratt & 

 Whitney Company of Hartford, one for their own use, and the other 

 the private property of Professor Rogers, used by him in his 

 professional work at Cambridge. It was by means of their compar- 

 ator that the Pratt and Whitney Company were enabled to establish 

 their well-known system of standard gauges. One great difficulty 

 in working with comparators, before they had reached their present 

 development, was the lack of sufficient and easily controlled 

 illumination for the microscope. This was, finally, ovei'come by the 

 use of the Tolles opaque illuminator, several of them being made, bv 

 their inventor, for Professor Rogers, and found to be thoroughly 

 adapted to his special work; and, at the present day, there is no 

 better, or more convenient, accessory to the microscope, for 

 illumination of opaque objects, under low powers, than this form of 

 illuminator. 



In 1880, under the direction, and at the expense of the 



