59i FOURTH REPORT — 1834. 



perhaps quite, as free from colour as the images produced 

 from reflection. 



To prove the durabihty of a glass thus constructed, the 

 author mentions the fact of a four-inch object-glass, which was 

 put together three years ago with Canada balsam, and has been 

 exposed to heat, cold, and solar light, without injury. In order 

 to remove a doubt recently start<;d on this subject, the glass 

 has been subjected to a heat of 140° for more than half an hour, 

 and immediately afterwards tried on the moon, when it ap- 

 peared to have suffered no injury. 



Considerable care must be observed in putting in the cement. 



It should be poured upon the centre of the concave lens, 

 over which the centre of the convex lens being let down 

 should be brought into contact with the cement, so as to pre- 

 vent the introduction of air bubbles ; the superfluous cement 

 is then to be gently pressed out, the pressure being applied at 

 the edges of the lenses. When this has been done, should 

 the lenses subsequently be shifted much, or turned round their 

 centres on each other, the distinctness of vision would almost 

 invariably be destroyed, and is not afterwards recovered. 



On a neiv Klinometer and portable Surveying Instrument. By 

 John Dunn, Optician, Edinburgh. 



[With a Plate.] 



Fig. 1. represents this instrument drawn to half its real 

 size. On the brass plate A B C D, there is traced a semi- 

 circle, divided into half-degrees, and within it a series of 

 rectangular coordinates, commencing at the centre. Round 

 the centre of the semicircle, an arm, fi F, moves, carrying the 

 sights ef, and a spirit-level L, turning on two pivots ; and at 

 one corner of the plate is placed a small compass-box, c*, re- 

 moveable at pleasure. The plate is attached to the tripod T, 

 fig. 2, by a universal joint, H S ; and the clamping-screws, 

 S and N, enable the observer to secure it in any required 

 position. 



To use this instrument as a klinometer, the edge A D is laid 

 on the dip of the stratum, and the arm E F is made horizontal, 

 by means of the level L, when the angle of the dip is indicated 

 on the semicircle by the edge I K, and the ratios of the base 

 to the altitude and slope of the inclined plane by the rectan- 

 gular coordinates, and the divisions on the straight edge I K. 

 For the more accurate purposes of the mining-engineer, and in 

 cases where the dip is to be determined over a considerable 

 extent of surface, the instrvnnent is placed in a vertical posi- 



1 



