324 MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



Illuminating and Observing Apparatus. 



The illuminating and observing apparatus consists of an autocollimating telescope, 

 mounted separately upon a stout pedestal similar to that which supports the 

 expansion apparatus, provided likewise with a very strong rack and pinion vertical 

 adjustment, in order to enable the telescope to be adjusted to the height required by 

 the particular deflecting arrangement mounted on the expansion apparatus. The 



Fig. 4. 



construction of the optical system will be more clearly evident from the section given 

 in fig. 4. Another view of the apparatus is given in fig. 5, so as to exhibit more 

 clearly the illuminating arrangement. 



The main optical tube, a, carries at the end furthest from the observer an achromatic 

 objective, 6, whose focus is at c, which is likewise the focus of the rays from the 

 illuminating lens, d, carried at the end of the side tube, e. At c is an iris diaphragm, /, 

 immediately behind which are a fixed diaphragm, </, pierced by a relatively large 



