Grubb — Some New Forms of Geodetlcal Instruments. 387 



deposited on the surface of the inclined glass (see p. 326 of the above-mentioned 

 paper on the gun-sights). 



The same effect as this last (superposition of two images) is produced in 

 many well-known instruments by a different device, viz., by utilizing one-half 

 the pupil of the eye for forming one image on the retina, and the other half of 

 the pupil for forming the other image on the retina. This is used in such instru- 

 ments as the Sextant, and by it observations of this superposition class (the only 

 reliable observations that can be made on shiji-board) are rendered possible. But 

 there are some objections to this jjlan — the eye requiring to be held very precisely 

 in one fixed position, the observations are difficult for an inexperienced jDerson, 

 and at the best they are eye-teasing and troublesome. 



By the use of the semi-transparent and semi-reflective films these objections 

 are removed. There is no necessit}^ for fixity of the observer's eye, and there is 

 no strain whatever in viewing the two objects and bringing them into coincidence. 



In one jDoint only does it appear that the old plan of '' dividing the pupil" 

 has any advantage, and that is in the possibility of varying the relative brilliancy 

 of the images by moving the eye slightly uj) and dowai so that more or less of the 

 area of the pupil is used in forming one or other image. This advantage, how- 

 ever, does not compensate for the disadvantages which necessarily accompany it, 

 as mentioned just now. 



It is not claimed that these instruments will ever supj^lant the standard 

 geodetical instruments, such as the Level and Theodolite, for their own legitimate 

 work, but those who have had experience in this line know that Theodolites and 

 Levels are often used for work for which they are not suitable, and in which their 

 accuracy and delicacy is not only wasted but is a positive disadvantage, and they 

 are used for this work only because there is nothing simpler available. 



It is for such work that I have designed these instruments. By their use 

 rapid surveys can be made by comparatively uneducated and inexperienced hands, 

 and with an accuracy as great as can be attained in plotting the survey on paper. 



For all ordinary surveying work, such as is necessary for road making, rail- 

 way making, and property-conveyancing purposes, the ultimate amount of 

 accuracy possible of attainment is limited to that with which the survey can be 

 plotted upon paper, practically the breadth of a pencil line. 



In the usual methods of surveying, the survey is plotted upon the paper either 

 from dimensions taken with the chain or angles measured with the Theodolite or 

 both ; but no matter how accurately these measures of distance and angles are 

 taken, the ultimate accuracy of the survey cannot exceed that above stated. 



With these new instruments bearings can be registered with a probable error 

 of less then two minutes of arc* This, in most cases, will be represented upon 



* According to most text-books an accuracy of a minute of arc should be obtainable. 



