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



ratchet round for the space of the tooth, thus causing the caoutchouc rollers to draw 

 the tape onwards for a corresponding space. The size of the ratchet teeth is so 

 arranged, relatively to the amount of movement given to the click, that the move- 

 ment of the ratchet and the tape does not occur until the punch has been extracted 

 from the tape. 



Readiny Telescope. 



A further accessory is a small reading telescope, by means of which the ther- 

 mometers can be watched by the observer without leaving his seat. This is mounted 

 upon a standard provided with the usual means of adjustment for height, altitude, 

 and azimuth. An inner sliding rod, capable of being arrested at any height by 

 means of a clamping screw passing through the upper boss of the fixed standard, 

 affords the means of adjusting the height of the telescope. Azimuth is arranged 

 for by attaching the telescope to a short tube sliding over the lengthening rod, and 

 fixable at any position by means of a slit and tightening collar carried at one end of 

 the tube. Altitude is provided for by the means of attachment to the sliding tube just 

 referred to, which carries a short projecting horizontal rod over which slips tightly a 

 corresponding short, narrow, horizontal tube, carried by the wider collared tube 

 through which the telescope is inserted ; rotation of the narrow tube over the rod 

 enables any desired tilt to be given to the telescope, or the latter to be arranged 

 precisely horizontally with the aid of a spirit level laid along the optical tube. A 

 further adjustment is attainable by means of a joint in the lengthening rod, which 

 enables the telescope to be brought to the most convenient position for the eye 

 when it is impossible to do so by moving the standard bodily. A strong clamping 

 screw is provided with the joint. At the summit of the lengthening rod is carried a 

 clamp which supports a large circular screen of dark-green silk, which serves the 

 double purpose of protecting the observer's eyes from the glare of the vacuum tube, 

 and interposing a broad non-conducting surface between him and the electrical 

 connections of the tube. The whole arrangement is seen in position in fig. 2 (p. 318). 



Accessory Interference Apparatus for Observing the Fizcau Phenomenon. 



The " Fizeau phenomenon " of periodical secondary interference, produced when 

 light not strictly homogeneous is employed for the generation of the bands, may be 

 very conveniently observed with the aid of the little accessory to the interference 

 apparatus which is shown resting on the movable slab of the expansion apparatus 

 in fig. 2, and which enables the separation of the two reflecting surfaces to be varied 

 at pleasure. 



It consists of a special base similar to the ordinary one (k of fig. 3), and adjustable 

 table (g of fig. 3) ; the latter carries three adjustable screws to act as an interference 

 tripod for the support of the large glass cover-wedge, and a platform, adjustable for 



