126 ANNUAL OF SCIENTIFIC DISCOVERT. 



NEW INSTRUMENT FOR MEASURING DISTANCES. 



Dr. Emsmann, in a paper in " Po<^gcndorfrs Annalen," de- 

 scribes a new instrument for measuring distances, whicih difVers 

 from all previous arrangements, by being independent of tlie 

 measurements of angles, or of a base line. It consists, simply, in 

 an application of the well-known principle, that the image of an 

 object is brought to a focus by a convex lens at a distance from 

 the lens varying according to the i-cmoteness of the object. The 

 aiTangemont descril)ed by Dr. Emsmann consists of an oliject- 

 glass of thirty seconds and an ej'e-piece of one S(!Cond focal length, 

 a screen of ground glass, upon which tlie image is received, being 

 placed behind the eye-piece. The instrument, it will be seen, re- 

 sembles in principle a photograph camera; the length, however, 

 is about five and one-half feet. In order to k<M'p the indications 

 within certain limits, tlie screen is placed beliind the eye-piece, 

 and the distance between the lenses is so arranged that a varia- 

 tion in the distance of twenty-five i)aces, at all ranges, requires, at 

 least, a movement of one line in tiie screen. Trustworthj' readings 

 may be obtained up to two thousand paces. Dr. Emsmann sug- 

 gests that the instrument will be found useful in coast batteries, for 

 measuring the distance of a vessel out at sea. In siege operations, 

 the time g(!nerally admits of the measurement of a base line, the 

 distance of the enemy's works l)t'ing calculated by trigonometry. 

 Should there be no practical diflieulties in the wa}% it might prob- 

 ably replace, with advantage, the stadiometer, which depends on 

 the priiicij)le of similar triangles, supplied the army for use in 

 judging distance-drill. 



THE CYCLOSCOPE. 



In places where railways are most needed, but whei'e, owing to 

 disadvantages of the ground, and other hindrances, the transport 

 and use of large instruments is very difficult, an instrument at 

 once portable, and capable of replacing a theodolite in settingout 

 railway curves, becomes a desideratum. An instrument called a 

 "Cycloscoi)e," or curve-tracing instrument, invented and patented 

 by Mr. H. Temjjle Humphreys, associate of the Institute of Civil 

 Engineers, is calculated to meet this want, by measuring angles 

 and setting out railway curves with increased facility. It may be 

 shortly described as an instrument combining the advantages of 

 a pocket-sextant with the principles of a kaleidoscope. When the 

 two jDlane mirroi's of an ordinary pocket-sextant are turned toward 

 a distant oljject, so that by one combined reflection between both 

 mirrors a reflected image of the object is obtained, the angular 

 inten'al between the image and the object is twice the angle con- 

 tained between the mirrors. Reijeated reflections of the same 

 kind would, of course, produce a series of images, growing dim- 

 mer, arranged at the same angle from each other as the first 

 image fi-om the object. This is found to be the case when the 

 object is indefinitely distant. When the object is near, as in the 

 common kaleidoscope, and placed between the mirrors, it is seen 



