22 



THE AMERICAN MONTHLY 



[February, 



bination of objective and ocular. 

 Hei'etofore it has been customary to 

 calculate the power of the compound 

 microscope by multiplying the power 

 of the objective by that of the ocular, 

 assuming the tube to be ten inches 

 long. This rule is founded upon the 

 supposition that the focal length of 

 the combination of objective and ocu- 

 lar is the product of the focal lengths 

 of objective and ocular divided by lo, 



fi Xf2 



or /"= , in which f^ = the 



-^ lo -^ 



focal length of the objective andy^ of 



the ocular. 



This would be true if the optical 

 tube-length were lo inches, but in 

 practice no distinction is made be- 

 tween the optical and the actual tube- 

 length. 



The meaning of optical tube-length 

 must now^ be explained. In fig. 3 

 the dotted lines f^ and f^ represent 

 the posterior focal plane of the objec- 

 tive and the principal focal plane of 

 the ocular, respectively. The dis- 

 tance from F^ to F^ is the optical tube- 

 length. 



If this length be represented by o 

 the true focus of the combination of 

 objective and ocular will be not 



/=--c^'but/=-^-^ 



The importance of this distinction 

 between actual and optical tube length 

 will be readily appreciated when it is 

 considered that with a ^-inch objec- 

 tive the focal plane is close to the 

 back lens, wrhile with a low^er power, 

 such as a i-inch for example, it is con- 

 siderably removed from it. 



The Ablbe Illuminator. 



Mr. J. Grunow, of New-York, 

 gives the following instructions for 

 using this illuminator as constructed 

 by him : — 



The apparatus consists of a lens- 

 system of very wide angular aperture, 

 two revolving diaphragm-plates, in 

 conjunction w^ith the plane and con- 

 cave mirrors on the stand proper. 

 The upper plane side of the lens-sys- 



tem should be almost even with the 

 upper surface of the stage, so that it 

 almost comes in contact with the 

 slide. For observation by central 

 light, the diaphragm with central 

 openings is used, viz., a narrower or 

 wider diaphragm, according to the 

 focal distance of the objective in use, 

 the nature of the object-slide, and the 

 intensity of the source of light. Gen- 

 erally, the narrowest diaphragm is to 

 be recommended, as it gives sufficient 

 light. Used without a diaphragm, 

 the condenser invariably gives an un- 

 satisfactory illumination. 



By moving the diaphragm openings 

 to the right or left, partly out of the 

 optical axis, oblique illumination is 

 obtained. 



For dark field illumination the star- 

 shaped diaphragms are used instead 

 of the aperture for central illumina- 

 tion, and always used in the central 

 position. At the saine time it is, 

 however, preferable to reduce the ap- 

 erture of all the high-power object- 

 ives, say froin one-fourth inch up, by 

 placing a diaphragm in the back of 

 the objective employed. The dia- 

 phragm is, however, to be taken out 

 again in every case when the objective 

 is used for transmitted light. Objects 

 not transparent cannot be view^ed by 

 this illumination, as the w^orking rays 

 of light have to pass through. 



The polariscope can be used in con- 

 nection with this apparatus. For this 

 purpose the condenser must have rooin 

 enough underneath the stage to have 

 an attachment for holding the polar- 

 izer. Polarized light can be used then 

 for central as well as oblique illumi- 

 nation. 



In using the condenser, the plane 

 mirror is generally used. Only when 

 viewing v^^ith very low powders, w^hen 

 the plane mirror does not completely 

 illuminate the whole field of view, the 

 concave mirror is used. In every in- 

 stance where the mirror is once ad- 

 justed for full illumination, the chang- 

 ing of the diaphragms does not affect 

 it. 



When using lamp-light, it is recom- 



