2 APPARATUS 



ever, which are not constructed in this way the arm is made to serve in lifting the 

 instrument. 



2. Always keep the microscope in its case or covered with a bell jar when not in 

 use in order to keep away the dust. A piece of black cloth supported on a wire 

 lamp-shade frame makes a most convenient protecting covering. 



3. Alcohol ruins the lacquer of an instrument and care should be observed to keep 

 all parts of the microscope from coming in contact with acids, alkalies, chloroform or 

 xylol as well as alcohol. 



4. Always use Japanese lens paper in wiping off the dust from dry objectives or 

 the immersion oil from the 2-mm. one. Should one neglect to wipe off the oil from 

 the oil-immersion objective, the dried oil can be removed by wiping with a drop of 

 xylol on lens paper, but the cleaning should be done as rapidly as possible, with a 

 final wiping off with dry lens paper, to avoid damage by the xylol to the setting of 

 the lenses. Throw lens paper away after using it once. 



5. Lenses are very liable to deteriorate in the tropics. One should be careful to 

 protect his instrument from the direct light of the tropical sun. 



6. If any oil is used on the mechanical parts for lubrication, all excess should be 

 wiped off to avoid the catching of dust or gritty particles. 



Objectives. To meet the demands of clinical microscopy there 

 should be three objectives, preferably a i6-mm. (%-inch), a 4-mm. 

 (%-inch) and a 2-mm. (J^-indi) homogeneous oil immersion one. 



The Zeiss A A is a ly-mm. objective, and the Leitz No. 3, an i8-mm. or ^-inch 

 one. The Zeiss D is about 4. 2-mm. and the Leitz No. 6, 4.4-mm. or ^-inch. A 

 dustproof quadruple nose-piece with four objectives will be found a great convenience 

 (in addition to the %-inch and ^2- mcn objectives, a ^-inch for urine and blood 

 counting, with a ^-inch for examining hanging-drop preparations and for quick 

 examination of blood smears). An apochromatic objective costs about three times 

 as much as an achromatic one and, except in photographic work, has little if any 

 advantage. 



Oculars. As regards oculars (eye-pieces) a Leitz No. i and a No. 4 

 will best meet the requirements. For high magnification a No. 8 may 

 be of service. The Zeiss oculars are numbered according to the amount 

 they increase the magnification given by the objective; thus a No. 2 

 increases the magnification, given by the objective alone, twice; a No. 

 8, eight times. 



Some oculars are classified according to their equivalent focal distance and are 

 referred to as H-inch, i-inch and 2 -inch oculars. 



A i-inch or 25-mm. ocular magnifies the magnification produced by the objective 

 about 10 times while a 2-inch or 5o-mm. one increases the magnification of the ob- 

 jective four times. 



A Leitz No. o is a 5o-mm. ocular and magnifies four times. The Nos. i, 2, 3, 4 

 and 5 are 40, 35, 25, and 20 mm. respectively and give eye-piece magnifications of 5, 

 6, 8, 10 and 12 times. 



The oculars in common use are known as negative or Huyghenian oculars, by 



