ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 237 



the interference bands, a little silvered reference ring in the centre of 

 one of the two surfaces reflecting the interfering light, and the micro- 

 meter spider-lines. There are two parallel vertical spider-lines ; one is 

 adjustable by the left drum-head of the micrometer, so as to be able to 

 set it at any convenient distance from the other in order to include a 

 single band and most of the reference ring between them ; and both are 

 moved together by the other (right) measuring drum, in order to be able 

 to determine the band-width and any fraction of a band which may 

 have passed the reference centre. 



The dispersion apparatus consists of a Hilger constant-deviation 

 prism, which enables the desired spectrum ray to be isolated from all 

 others, and that alone delivered to the interference apparatus. The 

 rays are deviated exactly at right angles by this prism towards the 

 interference apparatus, the surfaces of which they strike at normal in- 

 cidence, after which they return through the constant-deviation prism 

 (thus securing double dispersion) to the telescope. The prism is 

 mounted on a divided circle, so that it may be calibrated for the 

 delivery of light of any desired wave-length, if desired, and has numer- 

 ous adjustments. Such calibration is not essential, however, as the 

 particular image of the origin-stop in the colour corresponding to the 

 spectrum bright line of cadmium or hydrogen can be adjusted visually 

 on removal of the front lenses of the Ramsden eye-piece. 



The interference apparatus consists of three circular and thick glass 

 disks, the third of which is of black glass polished an absolutely true 

 plane on its outer surface, which is one of the two important surfaces 

 concerned in the production of the interfering light. It is ground on 

 the back surface, by which it is attached in an adjustable manner to the 

 right Microscope of the comparator, the movement of which it is to 

 record. The outer two are larger disks of colourless glass, identically 

 similar, the two truly plane surfaces of each disk not being strictly 

 parallel, but inclined at the minute angle of 35'. The left surface 

 of that one nearest to the black glass disk is the second surface con- 

 cerned in the interference, and approaches the black glass within a 

 millimetre ; the second is a duplicate one, merely introduced on the 

 right of it to correct for the slight dispersion produced by the 35' of 

 inclination, the two being set oppositely as regards the direction of the 

 wedge. The 35' inclination is just adequate to deflect out of the field 

 of the telescope the reflection from the other (right) surface of the left 

 colourless disk, and both images from the countervailing disk are got 

 rid of by a slight tilt in the rectangular direction. All the many 

 adjustments required are provided for in the mounting of the two 

 colourless disks on a separate carrier sliding along the face of the upper 

 V-and-plane bed of the comparator. 



The apparatus, as described up to this point, is the interferometer. 



The comparator consists of two V-and-plane beds, nearly 7 ft. long, 

 of specially homogeneous cast-iron, and worked truly plane with con- 

 summate care, together w T ith their contents ; they are arranged step- 

 wise, one on the top of the stone block, and the other 7^ in. below and 

 in front. On the upper one slide the two duplicate Microscopes, and on 

 the lower one the standard-bar carriage and accessory fine-adjustment 

 fittings. The carriage is given a longitudinal motion, a transverse 



