March 9, 1899] 



NA TURE 



439 



No one can doubt the great advantages of Dr. Wehnelt's 

 instrument in reducing the time of exposure of photographic 

 plates, the briUiancy and steadiness in the fluorescent screen, 

 not to mention its cheapness. The question of disadvantages 

 having been raised, such as control of the instrument, destruc- 

 tion of tubes, &c., I made a few experiments to see how far 

 such would affect its utility. My experiments were made with 

 induction coils of different sizes, the largest being a 2S-inch 

 spark, and in each case the principles involved seemed to be 

 the same. Firstly, we can vary the effects in the tube by in- 

 creasing or diminishing the voltage to the primary coil. .Secondly, 

 the current is easily controlled by difierence in proportion of the 

 size of the electrodes. Thirdly, the strength of the fluid in which the 

 electrodes are immersed affects the result. Fourthly, by the 

 distance between the electrodes in the electrolyte changes are 

 brought about. Fifthly, variations are got by the number of 

 active electrodes. By modifying these we can vary the length 

 of the spark, also its frequency and thickness, and it naturally 

 follows that we can make use of them in exciting Crookes' tubes. 

 In fact, so easy is it to arrange matters that I have been able to 

 excite small and large tubes of different vacua with perfect 

 safety even when using a large 28" spark coil. So far, there- 

 fore, from being afraid of its destructive power upon the anti- 

 kathode or Crookes' tubes generally, I am convinced that the 

 new interruptor will be very easily controlled. I may add that 

 some workers are inclined to think that large coils will no 

 longer be required, but the answer depends entirely upon 

 the work to be done. What we already do know, and what is 

 of greater importance, is that coils of any size can do what was 

 beyond their range in the past — a fact of great value where 

 portability is of consequence, as in hospital work or to the army- 

 surgeon on the field. 



So far I have therefore been quite able to confirm everything 

 promised by Dr. Wehnelt. Photographs taken by me of the 

 deep-seated tissues of the body have been obtained with very 

 much shorter exposures than by any other means of interruption. 

 Further, examination of these deep-seated structures has been 

 made much easier by the greater brilliancy and steadiness of the 

 fluorescent screen. 



Another question sure to be raised once more by this new 

 in.strument is which is the best instrument for X-ray work 

 judged by the comparative advantages and disadvantages of in- 

 duction coils and influence machines. Hitherto one of the great 

 advantages of the latter has been the brilliancy and steadiness 

 of the illumination of the screen, but this difference may be 

 said to h.ave disappeared with the advent of Dr. Wehnelt's dis- 

 covery. When the question of the best transformer was dis- 

 cussed at the Rrintgen Society last session, I could not see my 

 way to take any side, believing then, as I do still, that each 

 transformer has its advantages and disadvantages. It would 

 possibly be as easy to say whether a gas, oil, or steam engine 

 should be chosen to do a particular piece of work. At that 

 meeting I stated that when we come better to understand coils 

 and influence machines, we shall probably do the same work 

 with either. The selection of the instrument to excite Crookes' 

 tubes in the future will probably be decided by such questions as 

 portability, the primary force at the disposal of the operators, 

 such as batteries, street mains, or mechanical power. 



179 Bath Street, Glasgow, John Macintyre. 



March 5. 



To Calculate a Table of Logarithms. 



On the 23rd ult. you were good enough to publish for me a 

 short account of a method of calculating a table of logarithms 

 which any schoolboy might employ. I find that this method is 

 described in a book published a few days ago, "Measurement 

 and Weighing," by Edwin Edser, an associate of this college. 

 The method was used by Mr. Edser in teaching students of the 

 Chelsea Polytechnic. To find that young teachers are working 

 in the spirit evidenced by such an exercise, gives me many times 

 more pleasure than any that I could deiive from credit for 

 priority. John Perry. 



oyal College of Science, South Kensington, S.W., 

 March I. 



Probable Weather Conditions in Spain during the 

 Total Solar Eclipse of May 28, 1900. 



Thinking that some information about the climatological 

 conditions of the Spanish towns situated in the track of the 

 shadow during the total solar eclipse of May 28, 1900, may be 



NO. 1532, VOL. 59] 



welcomed by the readers of Nature, I have collected the few- 

 available observations made at meteorological stations situated 

 near the path. Only two are in the limits of the central 

 shadow : inland, Albacete ; and Alicante, on the sea-shore. 



Weather Chances towards the End of May, dtduced from 

 several Years of Observation. 



The Spanish astronomer, Mr. Landerer, has drawn up the 

 limes of contacts for the towns expressed in the following table. 

 (Madrid mean time.) 



Instituto Central Meteorologico, Madrid. 



A Remedy for Bookworms. 



Referring to the letter from Mr. J. Ewen Davidson on this 

 subject (Nature, vol. lix. p. 126). In my experience of 

 keeping books in Fiji and various parts of .-iustralia, I have 

 only had to contend with the attacks of rats, cockroaches, 

 " silverfish " [Lcpisma), and a small boring beetle ; all of which 

 attack the binding, probably for the sake of the starch, paste &c. 



The solution of corrosive sublimate, recommended by Mr. 

 Davidson, will undoubtedly ■^\o\t ^permanently effective without 

 the thymol. 



Some objection may, however, be taken to the use for this 

 purpose of a powerful poison like corrosive sublimate. 



I have attained the same end by giving the bindings a dressing 

 of strong shellac in spirit. This is easily applied by means of a 

 soft brush, and dries quickly, when it is scarcely noticeable even 

 on fine bindings. 



