626 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



placed on the stages of two different Microscopes arranged side by side. 

 The apparatus consists of a metal tube, and a prism casing containing 

 two adjacent and opposed right angled prisms, the latter being sur- 

 mounted bj the eye-piece. A prism is mounted at either end of the 

 horizontal tube. The course of the rays is shown in fig. 87. The 

 sockets of the prism tubes slip into the draw-tube of the Microscopes, 

 one being fixed by means of a clamping screw. Both Microscopes are 

 focused roughly before the comparison eye-pieces are put in position. 

 By means of the projecting stud the prisms below the eye-piece may be 

 displaced transversely for the purpose of viewing either the object on 

 the right or that on the left, or both may be viewed side by side and 

 compared. If polarized light be used the analyser is placed above the 

 eye-piece. 



Gordon's Diifraction Micrometer Eye-piece. — This micrometer eye- 

 piece, designed by J. W. Gordon, is of the type in which the read- 

 ing is obtained by means of a scale projected on the field of the 

 instrument (fig. 88). It is therefore a direct-reading micrometer, and 



Fig. 



possesses the advantage of a direct-reading instrument over those micro- 

 meters which have to be read upon the divided head of a screw. 



The scale is of special design, and as subdivisions can be actually 

 read and need not be estimated the scale is an open one, with very 

 clearly marked divisions, and is therefore more easy to be read than most 

 projected scales which are intended for accurate work. The extremity 

 of the object to be measured is ascertained by means of a micrometer 

 wire which traverses the field, the position of which is read upon the 

 scale. For the purpose of subdividing the scale divisions a diffraction 

 grating can be placed in position over the eye-piece, and will then, by 

 effecting the superposition of the image of the micrometer wire upon 

 the image of the adjacent scale-division, yield a reading of the fraction 

 of a scale-division over which the two images have to be moved in order 

 to bring them into coincidence. 



