69 



and those deviating considerably from the normal can 

 be used on objects for use with low powers only, in 

 which case the effect will not be very appreciable. 



"In this system I do not overlook the fact that var- 

 iation in tube length involves a variation in magnify- 

 ing power ; but, except in cases where micrometers 

 are used, I consider this of secondary importance, as 

 it always is in comparison to results obtained in resolv- 

 ing and denning power. 



"This system involves four conditions : 



First. That all cover glass be measured before 

 using them, and that the thickness be noted on the 

 preparation. 



Second. That for convenience all draw-tubes be 

 marked in inches or millimeters or both. 



Third. That adjustable objectives be corrected 

 according to this scale. 



Fourth. That the same tube length and cover 

 glass thickness be used in all original corrections of 

 objectives." 



To Draw Objects. It is very important that 

 the appearance of an object should be put upon paper, 

 especially of one which is not permanently mounted. 

 To do this does not require any great amount of skill 

 as the lines which are projected upon paper are 

 merely followed out ; but it is necessary that those 

 drawings be made truthful. Nothing should be put 

 down which is not actually seen ; neither should any- 



