388 HOW TO SEE WITH THE MICROSCOPE. 



notice th nature of one or two difficulties which may be said to 

 infest the observatory. 



We are all familiar with the purpose for which the transit 

 instrument is employed, namely, to observe the transit of celes- 

 tial bodies across the meridian in which the transit is adjusted. 

 To accomplish this, it is evident that the instrument must be 

 permanently placed in that meridian ; bnt, unfortunately, the 

 block of stone has yet to be found solid enough for the purpose. 

 Hence it is usual to ascertain the direction and amount of error 

 and thus correct the results obtained by observing with the 

 instrument while out of the plane of the meridian. The alti- 

 tude and azimuth instrument, too, requires frequent and careful 

 attention. To be assured that its object-glass may swing in a 

 vertical plane, observations are nightly made by observing a 

 distant star directly, and immediately afterwards its image as 

 seen in the mercurial horizon. By this means a truly vertical 

 line millions of miles in length is obtained. 



Reflecting instruments constructed on the model of the sex- 

 tant, are cross-questioned by observations on east and west 

 stars, and errors due to eccentricity thereby avoided. 



Not only do these painstaking men study their instruments, 

 but they in like manner study themselves, and their capacity to 

 observe is cross-questioned in the severest manner. It is to 

 them well known that the observations of some first-class 

 observers are constantly affected with a plus sign ; while, again, 

 those of other equally good observers are affected in a contrary 

 direction. This individual condition is called " personal equa- 

 ion," and has to be thoroughly known and compensated for; 

 and this you. must bear in mind, that the amount of error due to 

 personal equation is always comparatively small, and as a con- 

 sequence is only recognized in the observations of professional 

 experts, trained to machine-like, impassive regularity. 



My initial remarks, in which the kt Nasmyth's " membrane of 

 the books was discussed in a somewhat discursory manner, were 

 intended as an example, and to give you an illustration in refer- 

 ence to a radical evil, which has in times past tainted the obser- 

 vations of many who use the microscope I refer to diffraction 

 lines and diffraction borders. To avoid these spectral, illusory 



