2IO Anuruan Quarterly Microscopical Journal. 



oi the microscope it would seem better to modify, in some re- 

 spects, the requirements which should govern the construction 

 of a comparator. 



The ordinar}- method of measuring short lengths is either 

 by means of an eye-piece micrometer, in which the lines in the 

 focus of the ocular are ruled on glass, or with the ordinary 

 filar micrometer. There are quite a large number of microm- 

 eters of the latter class in use in this countr}'. The one 

 which I have used with great satisfaction was loaned to 

 me for some experiments by Mr. F. Habirshaw. of Xew York. 

 It was made by R. Sc J. Beck, and is excellent in every way. I 

 believe Powel <Sc Leland also make excellent micrometers of 

 this class. I am not aware that any American maker has given 

 attention to this important adjunct of the microscope. 



These two methods of measuring short lengths are both 

 open to two serious objections, both of which are inherent in 

 the method of construction and use. First, even with a ^4 ob- 

 jective one is limited to a field not pauch exceeding ^ mm.. 

 and secondly, in neither case can the contact of the lines of 

 the eye-piece with the lines under the objective be made in the 

 center of the field. 



These two forms of micrometers for measurements of short 

 lengths are. I think, the only kinds in use in this country. 

 Abroad, however, a micrometer screw which carries the object 

 to be measured in a plane perpendicular to the line of colli- 

 mation of the microscope is in frequent use. The screw has a 

 divided head, from which the magnitude of the space measured 

 is read directly. The writer has seen only the form made by 

 Merz & Son, of Munich. 



The introduction into this country of this form of a compara- 

 tor for short lengths is due to Mr. Leonard Waldo, assistant 

 in Harvard College Observatory. During his visit to the con- 

 tinent, in the summer of 1877. he purchased of Merz & Son 

 two of these comparators. One of them he attached to a 

 grand stand by Crouch, constructed upon a special order, with 

 reference to great stability. It is described in a paper on 

 ••Measures of Short Lengths," printed in the Proceedings of the 

 American Academy of Science and Art, Vol. XIII., page 352. The 

 other one was bought for the physical laborator}* of Professor 

 John Trowbridge, ^and was kindly loaned to me by him for 

 some exj>eriments. It was fitted to the Tolles stand, shown 

 in Fig. 8. It is marked A in the drawing. 



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