INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 947 



scope, " a department so largo that it might almost be regarded as a 

 new science, under the title of ' Microscopical Jurisprudence' " The 

 determination of hand-writing is more particularly referred to. 



The ' Bulletin Scientifique du Departement du Nord ' also contains 

 an article on the use of the Microscope in medicine, especially patho- 

 logical histology.* 



It is not possible usefully to give an abstract of either paper. 



Unit of Micrometry.— In his address mentioned above, Dr. Ward 

 says with regard to the resolutions of the Indianapolis Congress of 

 1878, that "as too often happens, their incidental faults attracted 

 more attention than their really scientific object. The unit proposed 

 (y^ mm.) was evidently too large for integers, and too short for 

 fractions, and unlikely to receive a single approval either at home or 

 abroad ; and the proposal of international action, though its object 

 was universally approved, was in a form not likely to accomplish that 

 object." 



The new committee appointed by the various Societies (see p. 154) 

 has not yet made its report 



Comparators for Measures of Length.!— Professor W. A. Eogers 

 has devised a comparator of the design shown in Fig. 1 to remedy 

 the defects found to exist in the eye-piece and filar micrometer as 

 well as in the Merz stage micrometer (A a). 



The comparator proper consists of a bed-plate, within which is 

 fitted a slide carried by the precision screw b. The object to be 

 measured is held in position upon the moving plate by the clips 

 shown in the figure. Instead of two parallel springs there is a single 

 cord attached to the centre of the moving slide whicli runs on the 

 guide pulley d, and is attached to a spring which is fastened to a 

 pin on the back side of the bed a little to the right of and below h. 

 The action of the spring, therefore, is wholly in the line of the screw, 

 and as the direction of the cord falls a little below the motion of the 

 slide, it has a slight tendency to keep the slide in contact with its 

 seat without introducing friction. The screw c moves the whole 

 bed-plate, including the precision screw h. The whole comparator 

 has a circular movement in the socket / attached to the original 

 substage e of the Microscope. The filar micrometer is shown at 

 h, and an eye-piece with a micrometer, having some advantages over 

 the usual form, is shown at i. Slow motion to the tube is given 

 through the lever g. 



The operation of using the comparator is as follows : — 



After the slide containing the graduations to be compared has 

 been placed in proper position under the objective, with the right 

 hand, the screw-head b is set at the zero of position ; with the left 

 hand, line 1 is brought in contact with a single line of the eye-piece 

 micrometer ; with screw-head b, line 2 is brought in contact with the 

 fixed line of the eye-piece micrometer, and the number of revolutions 

 and parts of a revolution are read off. Screw b is then brought back 



* Cf. ' Bull. Soc. Bclg. Micr.,' v. p. 243. 



t 'Am. Quart. Micr. Journ.,' i. (1879) p. 208. 



3 R 2 



