THE COMPOUND MICROSCOPE. 5 



respective lengths of the settings are so adjusted that when the 

 low power (which has an actual working distance from the micro- 

 object of about i inch), is in accurate focus, and, by means of the 

 nose-piece, is twisted out of position and replaced by the higher 

 power, this latter (which has an actual working distance l of about 

 -Jg- inch) shall also be approximately focussed ; while, if the nose- 

 piece is constructed with accurate centreing, an object in the 

 middle of the field of view with the low power ought to be also in 

 the field of view with the high power. Absolute accuracy in low- 

 priced instruments can hardly be expected, though this microscope 

 which we have selected as our type comes surprisingly near it ; 

 but after using the instrument for a little while the student will 

 find it possible to correct imperfect focussing, due to inexactitude 

 in the objective settings, by a ring or collar of paper, or possibly 

 even of thin wash leather, placed on the shoulder of whichever 

 objective proves by experience to be too short, before screwing it 

 into the nose-piece. 



The fine focussing (" fine adjustment ") upon the object is 

 effected by means of a micrometer screw, the large milled head 

 of which (M s) will be found at the top of the pillar. After a little 

 experience this should not be needed for low power work, the 

 sliding tube being wholly relied on. For work with the | inch 

 also the rough adjustment should be used as fully as possible, 

 though naturally with great care, and the micrometer screw fine 

 adjustment only used for exact final focussing. 



The student should as soon as possible get an exact mental 

 impression of the distance at which each objective stands from 

 the micro-preparation when in exact focus. When wishing to 

 focus upon a new object he should then place his head at the 

 level of the stage, and lower the tube carefully till he sees that 

 the objective is clearly within its proper focal distance. Placing 

 the eye to the eye-piece, and looking through the instrument, he 

 should then raise the sliding-tube gently, till he sees, either by 

 the object itself, or by marks on the cover-glass, etc., that he has 

 focussed properly. Focussing up is a great safeguard against 

 spoilt preparations ; and dirty object-glasses, even if nothing worse, 



1 As this actual working distance of the high power lens is calculated from 

 the object itself to the setting of the front lens of the objective, the amount 

 of this distance which is occupied by the cover-glass, i.e., the thickness of 

 this latter, is naturally of importance. With the low power this does not 

 come into practical consideration. 



