whether the power is sufficient ; it should be added 

 that for any over No. 14 and under No. 18 a 1 inch 

 eyepiece should be used and for those above No. 

 18 a power of f inch will probably be necessary. 

 After this condition has been complied with, look 

 to the correction collar of the objective. To obtain 

 the highest results it very often occurs that a dif- 

 ferent adjustment is required for obliqtie light 

 from that for central light. Note the number at 

 which the collar stands and then work it back and 

 forth, watching carefully for results. If this has 

 no influence, return it to its number or to a point 

 where the outline of the object appears most sharp. 

 Now look to the illlumination ; vary the distance 

 of the mirror to the object, or if this cannot be 

 done, vary the distance of the lamp to the instru- 

 ment and watch the effect of the change through 

 the eyepiece. If neither of these changes give any 

 improvement, recourse must be had to another 

 expedient. Place a bull's-eye between the lamp and 

 mirror with the plane side of the lens toward the 

 lamp and close to it, so that the light is thrown 

 on the mirror. It should be properly concen- 

 trated, so that the circle of light will not be larger 

 than the mirror, which can be determined by plac- 

 ing the hand or a piece of paper back of it. Adjust 

 when necessary by moving the lamp or bull's-eye. 

 Keep it a little below the line of the top of the 

 stage, so that the beam from the bull's-eye will not 



