1899J MICROSCOPICAL JOURNAL. 303 



be omitted to be said that owing to the introduction of 

 the new Jena glass, ordinary achromatics have been most 

 wonderfully improved and some of the most recent len- 

 ses have reached a perfection hitherto thought impossi- 

 ble ; such for instance as the new one-inch, by Ross, N. A. 

 •3 ; Leitz one-and-a-half inch and his tenth ; a new twelfth 

 by Beck ; No. 6 by Reichert, and others. This statement 

 is of importance to the student, but still there is no get- 

 ting away from the fact that by the use of apochromatic 

 objectives of the highest order, we verily believe'all the 

 great discoveries of the present and probably of the fu- 

 ture will be made. 



We have now to compare the performance of the achro- 

 matic and the apochromatic lenses visually and photo- 

 graphically. 



It is presumed the reader knows the value of employ- 

 ing what is called "critical liglit^ and how to obtain it ; 

 lest, however, such should not be the case, the following 

 is the method of obtaining it : Critical light is said to 

 be obtained when the substage condenser and the object- 

 ive are both in focus on the object. This is easily done 

 as follows : — first focus the object in the usual manner and 

 then rack the condenser up or down until the edge of the 

 flame of the lamp is distinctly seen in the field accurate- 

 ly focussed. To those unaccustomed to this class of il- 

 lumination it may appear objectionable, but no lens can be 

 said to be performing at its best except under these con- 

 ditions. The light, if the objective be of low power, may 

 be too intense ; if so, the Iris diaphragm may be closed 

 until the image loses its flooding of light, so long as the 

 image does not suffer in definition, because, cutting down 

 the light by the iris will lower the N. A. of the object 

 ive, but seeing that the low powers have much less aper- 

 ture than the high ones, a certain amount of contraction 

 of the iris will do no harm and will much improve the im- 

 age. Great care must be exercised, however, when at- 



