May, 1913. 



KNOWLEDGE. 



185 



divided screw head projecting backwards, which works on the 

 long arm of a " bell crank " lever, on the bottom or horizontal 

 arm of which rests the fine adjustment slide. The reaction is 

 taken up by a vertical spiral spring. The pitch of the screw 

 thread is forty to the inch. The vertical arm of the lever is 

 four times the horizontal. A complete revolution of the fine 

 adjustment screw gives, therefore, a movement to the body of 

 ,•00625 inches. 



The bearing surface of the fine adjustment slide is two and 

 a half inches in length, and the possible movement about one 

 eighth of an inch. 



The body is carried on a slide having a bearing surface of 

 three and a half inches in length, actuated by the coarse 

 adjustment pinion, with a movement of three inches. The 

 main tube is one and five-eighths inches in diameter, and con- 

 tains two draw tubes giving a latitude 

 of extension of from one hundred 

 and sixty to two hundred and sixty 

 millimetres. 



In the body tube, as close to the 

 back of the objective as possible, 

 are — first: an iris diaphragm ; second: 

 a slot which can be uncovered or 

 closed by a circular ring, and third : a 



slide carrying an analyser prism, Mb^l 



which can be moved in and out of wj 



the optic axis as desired. The two 

 latter accessories, in conjunction with 

 a polarising prism below the stage, 

 are indispensable for petrological 

 work. The advantage of placing the 

 analyser near the objective instead 

 of over the eyepiece — as is some- 

 times done — is that a much larger 

 field of view is obtained with a small 

 prism. The bottom of the inner 

 draw tube is screwed to receive a 

 " Bertrand Lens." Very perfect 

 interference figures in crystals have 

 been obtained with this instrument. 



At the bottom of the body is an 

 objective changer, which is preferred 

 to the usual nosepiece ; this is 

 considered as a fixture and is taken 

 into account when measuring the 

 tube length. 



A circular stage — five inches in 

 diameter — rests upon a stage bracket. 

 It is provided with centreing screws 

 and divided in degrees reading 

 from a fixed index. Upon the 



circular stage are two slides in which a mechanical portion 

 is held, having a rectangular movement of two and a quarter 

 inches from left to right and one and a quarter inches at right 

 angles. The stage can be completely revolved with the 

 mechanical portion in any position. The stage bracket is 

 connected to the bottom of the limb through a slide actuated 

 by rack and pinion, by which the stage can be moved in a 

 vertical direction with regard to the limb a distance of one 

 inch. The bearing surface of this slide is two and a half 

 inches in length and can be fixed by a binding screw in any 

 position. This movement of the stage adapts the instrument 

 to metallurgical work, and also allows for a variation of the 

 gap between the stage and body. 



The substage fitting is carried on a slide, actuated by rack 

 and pinion, having a vertical movement of two inches with 

 regard to the stage. It is provided with two carriers : the 

 upper provided with centreing screws to hold the condenser ; 

 and the lower to hold a polarising prism. These carriers are 

 supported on a vertical pin, upon which they can be separately 

 swung clear of the optic axis without dismounting — they are 

 provided with clamps to hold them in the central position. 



The tail-piece to carry the mirror is attached to the limb, 

 and is of the ordinary tubular type, which allows a mirror 

 two and a quarter inches in diameter to be placed in any 



Figure 182. 



Complete microscope, 

 structed by 



necessary position or to be swung out of the centre when 



using the instrument in an horizontal position. 



General : All the sliding parts of the instrument were 



designed to give ample bearing surface for the weight 



supported, and are all provided with means of adjustment 



and for taking up any slackness due to wear. 



The principal parts were cast in phosphor bronze and the 



smaller parts, bearing strips, and so on, were made of good 



quality brass. 



The optical parts and some accessories were supplied by 



Messrs. Watson & Son, as follows : — Objectives and oculars, 



" Holoscopic " series ; substage condenser, " The Universal " ; 



polarising and analysing prisms; objective changer, " Facility." 

 The total height of the instrument, racked down, without 



eyepiece, is fifteen inches. The weight, in working order, 

 = 17 lbs. 



The instrument can be easily dis- 

 connected into three parts. The limb 

 with all attachments can be lifted 

 from the stand by the withdrawal of 

 connecting pin in the inclining joint. 

 To prevent accident, the stand at this 

 joint is provided with semi-circular 

 cheek pieces, which support the limb 

 when the pin is withdrawn. The 

 stand can then be taken into two 

 parts — the vertical and horizontal — 

 which are connected by quarter inch 

 screws. When thus disconnected, 

 the instrument can be packed into 

 a case twelve and a quarter inches 

 by nine inches by five and a quarter 

 inches inside measurements. 



George G. Holmes, A.R.S.M 



PHOTOGRAPHY. 



By Edgar Senior. 



CRYSTALS AND X-RAYS— Of 

 the endless number of instances in 

 which photography has come to the 

 aid of the original investigator, none 

 perhaps are of greater interest and 

 value than the one which has recently 

 led to the discovery of the inter- 

 ference effects obtained with X-rays, 

 by means of crystals. Ever since 

 Rontgen rays were first discovered 

 in 1895, attempts have been 

 made to obtain results analogous 

 to interference, diffraction and reflection of ordinary light 

 waves, but always until recently with negative results. The 

 wave lengths of light visible to our eyes are those which are 

 comprised between 7594 (red) and 3930 (violet), and beyond 

 the violet are those of shorter length, known as ultra-violet, 

 to which photographic plates are particularly sensitive. Now, 

 it has always been supposed that X-rays were of the 

 nature of ultra-violet light of extremely short wave length, 

 quite beyond the limits of the spectrum known to us. The 

 experiments, however, carried out by Messrs. Friedrich and 

 Knipping at the instance of Professor Laue, opens up a 

 new range of the spectrum which was unexplored before. 

 In conducting any experiments by means of the spectrum 

 of visible light, a diffraction grating is very commonly 

 used in order to decompose the light into its constituents, 

 from which the wave lengths can be measured. A 

 diffraction grating is usually a plate of glass having a 

 number of fine lines ruled upon it at regular intervals, the 

 lines being practically opaque, acting as so many diffracting 

 objects, while the clear spaces between allow of free trans- 

 mission of the light. The spectra produced by means of 

 gratings are known as interference spectra. Now, as it was 

 supposed that X-rays consisted of waves of very short length, 

 Professor Laue formed the conclusion that it might be 



Designed and con 

 the writer. 



