120 JDr. Wollaston's Description of 



The construction that I have chosen for the scale is repre- 

 sented in fig. 1. (See Plate VI.) It is composed of small wires, 

 about tV of an inch in diameter, placed side by side, so as to form 

 a scale of equal parts, which may with ease be counted by means 

 of a certain regular variation of the lengths of the wires. 



The external appearance of the whole instrument is that of 

 a common telescope, consisting of three tubes. The scale oc- 

 cupies the place of the object glass, and the httle lens is situated 

 at the smaller end, with a pair of plain glasses sliding before 

 it, between which the subject of examination is to be included. 

 This part of the apparatus is shewn separately in fig. 3. It 

 has a projection at a, with a perforation through which a pin 

 is inserted to connect it with a screw represented at 6, fig. 2. 

 This screw gives lateral motion to the object, so as to make 

 it correspond with any particular part of the scale. The lens 

 has also a small motion of adjustment by means of the cap c, 

 fig. 2, which renders the view of the magnified object dis- 

 tinct. 



Before the instrument is completed, it is necessary to deter- 

 mine with precision the indications of the scale, which must 

 be different according to the distance to which the tube is 

 drawn out. In my instrument, one division of the scale cor- 

 responds to voooo of an inch when it is at the distance of 16,6 

 inches from the lens ; and since the apparent magnitude in 

 small angles varies in the simple inverse ratio of this distance, 

 each division of the same scale will correspond to two at the 

 distance of SyV inches, and the intermediate fractions toVo, 

 ToVo, &c. are found by intervals of 1 JSG inch marked on the 

 outside of the tube. The basis on which these indications were 

 founded in this instrument, was a wire carefully ascertained 



