82 



NATURE 



[October 14, 19 15 



or shell falls exactly where it can do most damage 

 to him. 



Range-finders have for their object to determine 

 this distance by some optical appliance. They are 

 divided into two classes : first, prism or base range- 

 finders, and, secondly, subtend range-finders. We can 

 explain the principle of these by reference to our eyes 

 and the method by which we roughly judge the dis- 

 tance of an object. When we look at an object the 

 optic axes of the eyes converge on it, and by long 

 practice we are able to appreciate the inclination of 

 the axes. The centres of the eyes are about 2\ in. 

 apart. Hence we have a very short-based isosceles 

 triangle, but we are enabled by our muscular sense 

 to give a rough guess as to the angles at the base 

 and practically to infer something about the length 

 of the triangle. Again, we do it in another way by 

 estimating the relative sizes of the image of known 

 objects, such as a man or house or other thing which 

 is formed on the retina. Another thing which assists 

 us is the amount of detail we see in the object 

 looked at. 



The range-finders used in war are only more exact 

 applications of the same principles. One of the most 

 accurate is that of Profs. Barr and Stroud. This is 

 a base or prism range-finder. It consists of a tube 

 varying from half a metre to two metres, about 6 ft. 

 in length. At the ends of this tube are two totally 

 reflecting prisms, which receive rays from the object 

 and send them down the tube. At each end of the 

 tube is an object glass, which forms an image which 

 is received on a peculiarly cut prism at the centre 

 and by an eye-piece. The arrangement virtually forms 

 a sort of double telescope corresponding to two eyes 

 set 6 ft. apart. When the observer looks into the 

 right eye-piece he sees a field of view which is divided 

 into two parts, one produced by light coming into one 

 object glass, and the other by that coming in at the 

 other. If the object seen is a mast, say, of a ship, 

 it appears broken in two parts. The observer can 

 rectify or bring into agreement these two parts of 

 the image by moving to or fro in the tube a thin 

 prism. The position of this prism is read off on a 

 scale seen with the left eye-piece. This scale shows 

 the distance in yards of the object. 



Thus on board our battleships a range-finder of this 

 kind is placed in one of the fighting-tops on the masts, 

 and the observer looking at a distant ship can in a 

 few seconds move the prism, adjust the two parts of 

 the Image to agreement, and read off the range. He 

 then sends down the range by telephone to the gun- 

 layers. Thus in the battle of the Dogger Bank, and 

 in that of the Falkland Islands, firing by our battle- 

 ships began at about 17,000 or 18,000 yards. The 

 range-finder would thus be continually sending down 

 the ranges 20,000, 19,000, 18,000 yards, etc., and the 

 gunners would keep the object vessel in sight and 

 fire when the command was given as the known 

 range of hitting was reached. 



The same principle is applied in a smaller instrument 

 for military use, called the Marindin range-finder, 

 invented by Major Marindin, only in the latter instru- 

 ment the means adopted for bringing the two parts 

 of the field of view or image into agreement are by 

 a movement of one of the prisms. 



The Barr and Stroud range-finder is a very accurate 

 instrument, and will determine ranges up to 20,000 

 yards, or about 12 miles, with an accuracy of 50 to 

 100 yards. 



In the next place there are range-finders called 

 subtend range-finders, which depend on the measure- 

 ment of the size of an image of a known object. 

 When we look at an object either with the eye or 

 with a telescope at different distances, it appears to 

 be smaller the farther away we are from it. In 

 NO. 2398, VOL. 96] 



the case of the eye we have no means of measuring 

 accurately this variation in size except by comparing 

 the apparent size of the distant object with some 

 near object the size of which is known. Hence judg- 

 ing distance by the eye requires long training, as 

 all sportsmen, sailors, and travellers know. 



Moreover, we are apt to be deceived as to the ap- 

 parent size. Ask anyone, for instance : How large 

 appears the full moon ? Many people would say, As 

 large as a shilling — meaning that it has the same ap- 

 parent angular magnitude as a shilling seen at 10 ins. 

 or I ft., which is the usual distance we hold a book 

 or paper when reading. 



But now, if you try the experiment, you will find 

 that the full moon is covered by a very small pencil, 

 like a pocket-book or dance-programme pencil, held at 

 10 in. from the eye. In scientific language, the ap- 

 parent size of the moon is about half a degree, which 

 means that it is covered by an object i/ioth in. in 

 diameter held i ft. from the eye. 



A man 6 ft. high would subtend the same angle at 

 a distance of 720 ft. Hence you can tell the distance 

 of a man by ascertaining the distance at which an 

 object of known size, say a pencil, must be held so 

 as just to cover his height. An ordinary pencil \ in. 

 in diameter held horizontally at arm's length ( = 2 ft.) 

 would just cover a man 5 ft. 8 in. high at a distance 

 of 544 ft., or 181 yd. The subtend range-finder works 

 on the principle of measuring the angular magnitude 

 of the object. One way of doing this is to place in 

 the focus of the eyepiece a plate of glass with divisions 

 ruled on it with a diamond. If we know how many 

 divisions are covered by an object of known height at 

 a known distance, we can tell the distance of any other 

 object of known height. 



It is very seldom, however, that we do know the 

 exact height of the object, and, moreover, it is very 

 difficult to count up accurately many very small 

 divisions ruled on glass when the object seen is at 

 all dark. 



During the vacation I have been turning attention 

 to methods for overcoming some of these difficulties. 

 As these inventions are being submitted to the Ministry 

 of Munitions, I do not think it desirable to go into 

 details as to the methods, but I will tell you the results. 

 I have invented three forms of range-finder — one which 

 is an improved subtend range-finder with which I can 

 find the distance of any object the dimensions of which 

 are known, whether height or width, or any part of 

 it. Also I have invented methods for using two such 

 instruments to measure the distance of objects the 

 dimensions of which are not known. In the second 

 place, I have invented a simple form of base range- 

 finder which measures what is called in astronomy the 

 parallax of any distant object, and hence deterrnines 

 its distance. In the third place, I have devised a 

 simple form of depression or elevation angle meter 

 by means of which the height of any hill, and also 

 the distance of any object from it, or from an elevated 

 position, can be determined by an observer standing 

 at the top of the hill, provided that he can also see 

 two marks placed at the base in line with the point 

 of observation on the hill and at a known distance 

 apart. These instruments are simple and inexpensive 

 to construct, and give an accuracy of measurement 

 quite sufficient to direct rifle or artillery fire or bomb 

 throwing in trenches. One great advantage of my 

 range-finder is that it can be used with a periscope 

 from the bottom of a trench so that the observer need 

 not be exposed at all, but can determine the distance 

 of the enemy's trench by observation on any post of 

 a wire entanglement or stick or rock or anything with 

 a sharp outline. Another principle which may be 

 applied in making a range-finder, which I have also 

 done in my instruments, is to observe the variation in 



