TEST OBJECTS. 481 



a means of obtaining the finest performance under various 

 circumstances. The following directions are thus given for 

 their use: — 



1st. When the tube in the body of the microscope is not at 

 all drawn out. 



If the object is uncovered, screw up the collar of the object- 

 glass, till stands opposite to the vertical mark on the tube, 

 its two or more horizontal marks, each of which indicates one 

 revolution of the collar, being all fuUy exposed. (This is nearly 

 as far as the screw will go without strain.) 



If the object is covered with glass or talc, measure the thick- 

 ness 'of this, taking advantage of dust or spots on the surfaces, 

 by the milled head for slow motion: it has its circle divided 

 like the collar of the object-glass from to 9; every revolution 

 being ten divisions. 



Multiply the number of divisions indicating the thickness 

 by 0.7, it the \^ inch object-glass is used; by 0.9 if the ^ inch. 

 Then set the coUar to the number that is the product, screw- 

 ing it down from its former position, and pressing up the tube 

 of the front lens; and the adjustment is made. 



2nd. When the tube in the body is drawn out. 



Increase the number to which the collar is set, with the 

 -^ inch glass, as under: — 



For 1 inch drawn out add 2.5 divisions. 



2 inches . . 4 ditto. 



3 ditto . . 5 ditto. 

 5 ditto . . 6 ditto. 



The \ inch glass is little changed by lengthening the tube, 

 but one division may be added for each of the first four inches 

 drawn out. 



* * The milled head for slow motion gives for the depth of 

 of an inch in air fifteen divisions, in glass ten nearly. 



iffir 



In order to test the merits of an object-glass, an object 

 suitable to its powers should be employed; if below the half- 

 inch no achromatic condenser need be used by day; the light 

 from a white cloud may be reflected by the mirror, or that 

 31 



