April 24, 19 19] 



NATURE 



^47 



arranged for the prism to be capable of easy 

 removal for cleaning purposes or for renewal. 

 An additional advantage of this method of con- 

 struction is that a series of prisms may be em- 

 ployed, giving further ranges of refractive indices 



-Dipping Refractomeler (Kellingham and 



Stanley, Ltd.). 



up to I 55, with an accuracy of three or four 

 units in the fifth decimal place. 



For measurements of still higher accuracy, the 

 Pulfrich refractometer is available (Fig. 4). In 

 ordinary use this instrument will give results four 

 or five times as accurate as those obtained by 



a u o 



111.. ... — I';,;;i. ;. KelVactometer (,\dam Hilger. I.t-i 



objcct-gldn.-^ auil prism dust cover; j, telescope :.v. .„^,.., ....^ 



E.P. ; K. prism for use with sodium burner ; l. coadeu.'.cr 

 height-adjusting milled head ; m, bottom water jacket with 

 Pulfrich prism ; .s', top water jacket ; o, thermometer case ; 

 V, circle .slow motion (position only indicated) ; (}, slow 

 motion vernier (position only indicated); R, slow motion arm 

 clamp milled head ; s, clamp screw for bottom water jacket ; 

 T, light screen ; u, thermometer adapter case ; v, thermometer 

 ca.se operating milled head ; w, clamp for vacuum tube holder. 



means of the Abbe refractometer. Mr. J. Guild, of 

 the National Physical Laboratory, claims that, 

 with proper care in design and use, the Pulfrich 

 refractometer will give results accurate to the fifth 

 decimal place, not only in the dispersion, but also 

 NO. 2582, VOL. 103] 



in the absolute index. In this instrument the 

 substance to be examined is placed on top of the 

 horizontal surface of a block of glass of known 

 refractive index. Rays entering the substance 

 from one side can pass out from the opposite 

 vertical surface of the Pul- 

 frich prism only when they 

 enter above the horizontal 

 boundary surface. A sharp 

 line representing the rays 

 which have just been able 

 to enter the prism is 

 observed in the telescope. 

 The angle of emergence is 

 measured by rotation of the telescope, which is 

 attached to a divided circle. Messrs. Hilger have 

 designed a new instrument in which all screw- 

 heads are brought within reach of the observer's 

 right hand. Direct readings on the vernier of the 

 divided circle are accurate to one minute, and on 

 the divided drum of the slow motion to six 

 seconds. In accurate measurements the questions 

 of temperature control and of the source of light 

 i employed must receive careful consideration. 



I __. 



THE ATLANTIC FLIGHT. 

 T^HE first attempt to cross the Atlantic by aero- 

 j -■- plane will go down to posterity as one of 

 ! the milestones in the progress of aviation, and 

 there seems little reason to doubt that this feat 

 will soon be accomplished. The two main factors 

 affecting the result are the trustworthiness of the 

 engine and the state of the weather. The best 

 engines of to-day are capable of running for 

 periods considerably longer than that required for 

 ' the crossing, and, although it is impossible to say 

 that a given engine will accomplish a twenty-hour 

 run without mishap, the chance of failure due to 

 engine breakdown is by no means exceptionally 

 great. On the other hand, the weather is ex- 

 tremely diflRcult to forecast, and very little in- 

 formation is obtainable as to the conditions pre- 

 vailing at a height of 10,000 ft., even though the 

 I surface conditions are fairly well known. Every 

 I possible provision will be made for the safety 

 of the aviators in the case of a forced descent at 

 sea, but the element of risk is naturally a very 

 serious one, and we can but admire the men who 

 are so ready to face it. 



Mr. Hawker, on his Sopwith machine, is carry- 

 ing a collapsible boat, attached to the upper side 

 \ of the fuselage, containing signalling devices and 

 ; provisions for two days. Even with such a precau- 

 j tion the risk would be very great in a rough sea, 

 I and the chance of attracting the attention of ships 

 j would be small. It is understood that Mr. 

 Hawker will not be able to send, but only to 

 receive, wireless messages. This is unfortunate, 

 : for in the event of a forced descent the machine 

 would take about ten minutes to glide from a 

 height of 10,000 ft., and there would be ample 

 time to get into communication with any vessels 

 in the vicinity. It is intended to drop the under- 



