32 



case the real aperture of the telescope is found by moving the pointer over the object- 

 glass until its point is just visible, and measuring from the inner edge of the brass 

 cell holding the object-glass to the pointer. Twice this distance subtracted from 

 the distance between the two edges of the brass cell, will give the real or clear 

 liprrture of the telescope. The clear apert ure, divided by the diameter of the small 

 circle of light at the eye end, when the telescope is focussed on a distant object, 

 will give the magnifying power of the telescope. Thus the clear aperture of a tele- 

 scope, measured by means of a pair of dividers and a scale, was I 1 ?- 35, while tho 

 diameter of the circle of light at the eye end, was, 0'?- OG. In this case, the magnify- 



ing power of the telescope was -~- = 22.5 diameters. 



Another way to determine the magnifying power, is to measure the angular 

 distance between two points with a transit, and then measure the same distance 

 with the telescope of which the power is to be ascertained, placed so that the tran- 

 sit must point into its object-glass and see the same angular distance through the 

 second telescope inverted. Then calling the first angle A, and the angle as seen. 

 diminished through the introduction of the second telescope inverted a, we have 



the magnifying power of the second telescope =- an ' -. Thus the angle sub- 



tended by a window sash, several .hundred feet away, was measured by a transit 

 instrument direct, and found to be, 158'50". When a Y level, previously focussed 

 on a distant object, ws set before the transit, with its object-glass towards tli* 

 transit, the same sash \vaS measured and the angle was found to be but 3' 30". In 

 tliis case, therefore, 



/15S'50"\ 



tcin. I -- ^ - I tan o50 / 25 v 

 the magnifying power of Ylevel= -- ///\ ^tun l^o"^ 34 ' diamctcrs -- 



Or, for an approximation, a card cut one inch wide may be set up across a roori 

 by the side of a measure graduated to inches. Then, the number of inches on the 

 measure seen by one eye, covered by the image of the white card seen through the* 

 telescope by the other v?ye, will give, roughly, the magnifying pow r er. 



It is difficult, without months of use, to fully test an instrument in all its parts ; 

 but in choosing an instrument the engineer should bear in mind that the making of 

 the transit ana the level are considered to be feats of mechanical skill. It should 

 be remembered that there is no machine so delicate that it can finish the essential 

 parts of an instrument. The last stages in its making must depend upon the personal 

 skill of some mechanic, who has a reputation for that particular work ; and we an. 1 

 sorry to add, that so difficult is it to secure the mechanical skill and patience re- 

 quired in the finishing of the interior pails, the only essential ones, and so easy ia 

 it to add the lacquer and polish of the outside, that the market is full of instruments 

 sold at a price enough lower than the best makers can work, to seem to eflcct i 

 large saving of the lirst cost; but such a saving is money borrowed at the highest 

 rate of interest, when the cost of annual repairs is considered. It is better at t ho 

 out-et to buy of a maker who is noted for the conscientious accuracy of his work. 

 An imperfect raek motion; a screw turned home on the wrong thread : a wabbling 

 of the object-slide or eye-piece; a slight space between the edge of the vernier ana 

 the limb of the circle; in fact, any mechanical defect, no matter how slight it mar 

 seem, may be taken as a pretty sure indication that the work has been slighted in 

 other parts as well, and should have a strong inl'.uenee in guiding the selection of 

 an instrument, in the absence of a test by work in the field. 



Th.e Engineer's Transit. 



In tin- first part of this catalogue, Messrs. C. L. BEIU.I i; A SNS. have pointed out 

 the peculiarities and improvements in this instrument, as constructed by ther.i. In 

 Fl>e;,king of the adjustment* of these instruments it is well for the engineer to 

 remember that the construction is aimed to be such that if the telescope and levels 

 are can-Cully adjusN-d they may remain so for even a number of years to come, if 

 the instrument HinVrs no rough usage. 



