I04 



NATURE 



[October ii, 19 17 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Modern Range-finder. 



Prof Boys, in his review of Prof. Cheshire's 

 pamphlet on "The Modern Range-finder" in Nature 

 of September 6, has raised certain questions which 

 may be amplified with advantage. 



The application of stereoscopic principles to range- 

 finding is due to Hector Alexander de Grousilliers, 

 who communicated his invention to Messrs. Carl Zeiss, 

 by whom it was elaborated and constructed. As in 

 principle the stereoscopic range-finder is so simple and 

 beautiful it is desirable that the name of the true 

 inventor should be remembered. 



Prof. Boys is scarcely correct in stating that 

 the stereoscopic range-finder was proposed by a 

 workman in the Zeiss works. De Grousilliers was a 

 chemist and an engineer in the Continental sense, who 

 lived at Charlottenburg. His British patent, No. 17048, 

 was applied for on September 11, 1893. ^^ is interest- 

 ing to note that the corresponding German patent, 

 identical in substance, and applied for on January 3, 

 1893, is issued in the name of Messrs. Carl Zeiss. 



As Prof. Boys says, it is fascinating to sweep the 

 scale past more or less distant buildings and see the 

 divisions of the distance scale pass behind or in front 

 of the different objects; but when the observations are 

 made upon objects of known ranges and the results are 

 analysed, the fascination is generally tempered by dis- 

 appointment. 



It has been said with considerable truth that a 

 coincidence observation is a fact, whereas a stereoscopic 

 one is an impression ; the former is based upon a self- 

 contained micrometer measurement effected by one eye, 

 while the latter is the result of balancing the effects 

 produced in two separate eyes. 



Prof. Boys suggests that for anti-aircraft work the 

 stereoscopic range-finder may be of advantage, owing 

 to its speed of operation, on the assumption, presum- 

 ably, that an object in any part of the field can be. 

 compared with the fixed scale and that accurate direc- 

 tion of the instrument upon the target within the 

 limits of the field is therefore not required. In prac- 

 tice this is not the case. If any reasonable accuracy 

 is to be obtained, the object must be brought close to 

 the appropriate mark or on to the imaginary "plastik" 

 line between two marks ; that is to say, the' instrument 

 must be both trained and elevated with considerable 

 accuracy. In a coincidence range-finder the image 

 must be brought to the separating line, but it may 

 occupy any position along the length of the line. 



In a stereoscopic instrument it is necessary to compare 

 the image with one mark, then with the neighbouring 

 one, and, finally, to locate its position between them. 

 Compared with coincidence, the .stereoscopic observa- 

 tion is not more speedy, and therefore not more suit- 

 able, for anti-aircraft or similar services, where speed 

 of operation is essential. It involves as careful direc- 

 tion and the accuracy attained is much inferior. Great 

 claims as regards accuracy have been made for stereo- 

 scopic range-finders, but these have not been substan- 

 tiatecj in practice. 



With regard to the question of accuracy, the resolv- 

 ing power of the objective is not one of the limiting 

 factors in the case of coincidence observations. Sup- 

 pose, for example, that the image is a point circle; 

 then the coincidence operation consists in making the 

 NO. 2502, VOL. 100] 



separating line cut the circle approximately diametric- 

 ally, and in bringing the edges of the upper semicircle 

 into alignment with those of the lower. If now the 

 resolving power of the objective is diminished by de- 

 creasing its diameter, the point image will be of larger 

 diameter, but the coincidence operation will not be 

 more difficult than before. Indeed, if the objective 

 diameter is so small that the image is surrounded by 

 distinct diffraction circles, the operation will be facili- 

 tated, as such lines constitute ideal coincidence objects. 

 The accuracy depends upon the character of the edges 

 as regards sharpness, and not upon the size of the 

 image of the ideal point. 



The coincidence observation figures quoted by Prof. 

 Cheshire are quite ordinary. So far as the unaided 

 eye is concerned, the only limit to resolving power 

 that the writer is aware of is the quality of the 

 definition of the edges of the image at the retina. On 

 natural objects the unaided eye can resolve less than 

 four seconds, and under good conditions of definition 

 an accuracy of two seconds has been obtained with 

 considerable consistency in coincidence observations, 

 but phis or m.inus three to four seconds is the more 

 usual practice. James Weir French. 



Anniesland, Glasgow, September 27. 



I AM obliged to Mr. French for correcting me in the 

 matter of the origin of the stereoscopic range-finder. 

 My statement that the idea originated with one of 

 Zeiss 's workmen was made on the basis only of my 

 recollection of conversation with Dr. Czapski at the 

 Paris Exhibition in 1900, and it may well be that my 

 recollection is at fault, or possibly that I misunderstood 

 what I was told. 



When suggesting that this type of range-finder might 

 have some advantage for anti-aircraft work, the ob- 

 servation I had in mind was sweeping the scale of 

 distance slowly across the object and noticing which 

 division came within and which beyond, and then not 

 more than a rude guess at the proportion between. 

 Such a process I considered would give a very quick 

 but less accurate range than that given by a coinci- 

 dence instrument, but nevertheless a very useful range 

 in view of the rapid and erratic change of distance. 

 The most aggravating property of the stereoscopic 

 instrument is the transparency that it imposes, even 

 upon buildings, for the moi-e distant scale divisions 

 remain in view as they pass behind them. 



C. V. Boys. 



A Plea for the Fuller Utilisation of Coal. 



The important letter from Major Martin (Nature, 

 August 16) on the above subject involves many con- 

 siderations, and I would suggest that the following 

 points merit attention :' — 



(i) It seems probable that after the war there will be 

 a demand for greater home comfort among the poorer 

 paid classes of the community. The supply of very 

 cheap gas for heating and cooking should improve 

 matters greatly. 



The cost would be further reduced if arrangements 

 could be made to fit houses with surface-combustion 

 heaters, cookers, and the like (cf. Prof. Bone's experi- 

 ments). 



At present it appears to pay many gas companies to 

 supply ordinary gas stoves on specially favourable 

 terms to consumers, so that there seems no reason why 

 surface-combusters should not be supplied from vari- 

 ous depots in different districts. If the gas were 

 distributed at a high pressure, it should be possible 

 to devise some injector arrangement which would 

 obviate the necessity for the use of compressed air, a 



