ggO SURVEY OF THE COAST 



of these pieces at about 120 degrees from each other. The 

 vertical adjustment of the whole microscope to the proper 

 focus is made by two milled rings embracing the micros- 

 cope, one above, the other below, the piece q. The adjust- 

 ment for the arrangement of the microscope, as it refers to 

 the proper situation of the wires with respect to the focus of 

 the object glass, is obtained by screwing the lower concealed 

 part into or out of the upper cylindrical part. It is held in 

 its proper place by a small ring b'. As it is well known that 

 these two last mentioned motions must always be made to- 

 gether, and the clearness of vision, absence of parallax, and 

 accurate measurement must be obtained by both at the same 

 time by trial, it will be sufficient here merely to indicate the 

 screw serving for it. 



A small screw pp^ at the end of the micrometer opposite 

 the screw head moving the rake or indented plate of the 

 same which indicates its 0° point, will make the final adjust- 

 ment to the division point, or the distance between the mi- 

 croscopes desired. 



After some experience, I have, however, found it best not 

 to adjust the microscopes to 130° distance, as it appears to 

 me that the influence of a little deviation in this respect may 

 be wholly neglected in comparison with the advantages of 

 having the microscopes fixed simply in the middle of their 

 holding pieces qq, which makes them more firm, and places 

 the 0° point of the micrometer in the middle of the field of 

 vision. 1 used them therefore always in this position, in 

 which they remained perfectly steady, and which gave be- 

 sides a kind of moral advantage, from the circunislance, that 

 as the readings became thus different in minutes and seconds 

 for each microscope, the observer remains entirely unpreju- 

 diced as to what he should read at each microscope, and 

 each reading becomes thereby equally independent and im- 

 partial. 



Upon the drum i are two spirit levels, four inches long, at 

 right ansjles to each other, to serve for the first approximate 

 levelling of the instruments, which 1 found, under tolerably 



