THE SMITHSONIAN INSTITUTION. 



255 



besides, necessary for night observations ; tlie lamp being placed before 

 the instrument and above the eye, to reflect the light. 



Fig*. 



The method of reading oif will perhaps be best explained by a fevv^ 

 examples. Suppose, after completing the adjustments,^ the scale and 

 vernier to be in the position shown in Fig. 4, on this page, it Avill 

 be seen that the lowest or index line of the vernier coincides exactly 

 with the line marked 30 on the scale. The reading, therefore, is 

 30.000 inches. 



If, as in Fig. 5, we find the line of the vernier coinciding with the 

 third line of the tenths above 29, we read 29.300. 



