S76 SVRVEY OF THE COAST 



half lenses may be in the direction of the base or bars, in 

 order that the light from both objects may be as equal as 

 possible, which would not always be the case in any other 

 position. The microscope must of course be placed at the 

 proper distance from the point /i, so as to present a distinct 

 image of this point. 



It is evident, that by this arrangement the optical contact 

 obtained is similar to that of the microscope reading on a cir- 

 cular instrument, and can be made with great ease and ac- 

 curacy. The determination of the point of contact in the 

 veitical direction is equally accurate 5 fur the limit of only 

 tolerable vision in the compound microscope, which for the 

 cobweb has about three inclies focal distance, is already very 

 narrow, and within less than one-twentieth of an incii. This 

 would hardly aftcct the level of the bars, and still less tlie 

 difference between hypothenuse and base. But when the 

 vision is carried to distinctness, it will be totally destroyed. 



The two horizontal motions required to place the point 

 h, and the microscope, are made, in the direction perpendi- 

 cular to the bars by the screw, kk^ revolving in the end pieces 

 of the plate, ii, and working in the soLk'>t which projects 

 above tiie middle of the lower plate, jy. In the direction of 

 the bars, the motion is made by the screw, W, which turns 

 in the lower plate, jj^ and works in the socket projecting 

 fiom the circular piece, ?«;«, below. '1 he positions of these 

 screws, wlien in actual use, are seen in Plate III. fig. 7 ^' 8. 



The circular plate, mm, is encompassed hy a ring. «;?, 

 which presses it down to the plate, 00, to which this ring is 

 fastened, but in such a manner as to admit its entire revo- 

 lution within them, by hard friction, as it is usually called. 

 By the circular revolution of this plate, mm, the whole up- 

 per part of this apparatus can be placed in tlic |)osition I)est 

 adapted to receive the bars freely, as is represented in the 

 figure. 



The plate, 00, is about four inches and a half in diameter, 

 and is fastened to the steel triangular bar, pp. wiiich is about 

 seven inches long, and which slides closely up and down \u 



