Accessories, 51 



the object, and by this means light at any angle could be 

 reflected on to the stage. The day for this, however, has 

 gone by, and anyone who requires to get even fair results 

 must use a sub-stage condenser in some form or other. 

 Especially does this apply to high-power objectives. Plenty 

 of illumination can be obtained with the mirror only for 

 lenses up to, say, J-inch, but beyond this the object 

 becomes ill-defined and the field dark. More especially 

 since the study of bacteriology has taken so prominent a 

 position has the condenser come to the front. Without its 

 aid it would be almost impossible to distinguish between 

 these minute organisms. The condenser introduced by 

 Prof. Abbe (of the firm of Zeiss, Jena), and termed the Abbe 

 illuminator, is that most popularly in use for this purpose, 

 and fulfils all ordinary requirements. It is made in two 

 forms, having numerical apertures of 1*20 and 1*40 respec- 

 tively. The former is most generally employed, and can 

 be obtained at a very low price. On account of its large 

 aperture it gives a most brilliant illumination with the 

 highest powers, while with the lower powers, by removing 

 the top lens, equally good results may be obtained, its only 

 drawback being that it is not achromatic. It has been 

 claimed by some that this is not an essential to a sub-stage 

 condenser, but a comparison of this with the new achro- 

 matic form of the same maker certainly gives the advan- 

 tage to the achromatic condenser, but it has not such a 

 large aperture as the chromatic form. The chromatic form 

 of condenser is supplied by nearly all the London micro- 

 scope-makers, but the new achromatic form having a 

 numerical aperture of I/O is made by only two or three 

 firms. We should strongly advise purchasers to have 

 these condensers of English make, as the German mounts 

 are so exceedingly heavy, and, not being usually intended 

 for the English form of microscope, have to be adapted, 

 and where they are made by German firms for the English 

 microscope arc so clumsy that they are apt to impart 

 vibration, and are awkward to handle. 



