ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 457 



of the image is in the constant ratio /2//1 to that of the object for every 

 value of H, inasmuch as the distance from these to the principal points 

 gives the measure of the " penetration " of the system. 



Campbell's Micrometer-Microscope. — This (fig. 96) was originally 

 devised by Sir Archibald Campbell for measuring photographs of spectra ; 



Fig. 96. 



it has since been improved for measuring diffraction gratings, and special 

 means have been added for recording end-measurements of standard gauges 

 by utilizing electrical contacts. It is made by Mr. A. Hilger. 



It consists of a horizontal metal frame, in which a Microscope is ap- 

 plied to slide over a space of 5J in. actuated by a micrometer-screw. 

 The frame is supported on three j)illars, with adjusting screws for levelling 

 with conical ends fitting in V-slots converging to a common centre, and 

 applied on a substantial iron base-plate standing on adjustable screws, also 

 for levelling. 



The micrometer-screw has a pitch of 100 threads to the inch; the 

 drum-head connected with the screw is divided into 100 parts on the edge, 

 and by means of a vernier, direct readings can be taken up to 1/100,000 

 of an inch. For registering entire revolutions of the screw a fixed scale 

 corresponding with the pitch of the screw is engraved on one side of the 

 frame, and an index-pointer travelling with the Microscope gives the readings. 



The diffraction gratings, &c., are carried on an adjustable stage with 

 mirror that can be placed as required on the base-plate under the Microscope. 



1887. 2 H 



