218 ON THE ABSOLUTE MEASUREMENT OF HARDNESS. 
to be observed, of course, for an invariable position of the plate and lens 
with reference to the horizontal, and the measurement is made through 
a microscope with its line of sight normal to the plate, seeing that the 
lengths to be taken are small. In the field of the microscope the im- 
pressed area appears in form of a dark circular spot, which, together 
with the rings surrounding it, presents a case of interference. I shall 
show that even the diameters of the rings are available for measure- 
ment. Further particulars however are best discussed in connection 
with the apparatus. 
IV. APPARATUS. 
Through the kind permission of Prof. Abbe, the apparatus was 
constructed in the workshop of M. Zeis, of Jena, and i desire in this 
place gratefully to acknowledge the suggestions received from my col- 
leagues, in particular from Prof. Abbe, during the course of its con- 
struction. Fig. 1 shows the completed instrument in sectional eleva- 
tion, nonessential parts having been withdrawn for clearness. It is 
put together massively, so as to withstand the powerful stresses which 
are to be brought to bear on it, and it is firmly planted on a pier in one 
of the vaults of the university. Ample provision is made to guard 
against tremors. The cast-iron bedplate G Gis T-shaped in eross 
section, 73°" long and 7.5°™ wide, and a central gutter runs from end 
to end. 
The support 7, screwed to the bedplate, is provided above by a re-en- 
trance ¢, in which the knife edge D, around which the wrought-iron lever 
H H’ is free to turn, is suitably adjusted. The short arm of H H’ ter- 
minates in a ring-Shaped expansion / 1’, at a mean horizontal distance of 
about 5°" from the axis P. In the conical perforation in Ul’ a plug Z 
fits snugly and the lens Z is attached to the top of Z The other arm 
of HH’ is about ten times as long as the short arm, and ends in the 
knife edge c. The glass plate p, to be tested, is attached to the upper 
perforated plate oo’, secured by means of a pillared arrangement, of 
