VISUAL WORK WITH THE MICROSCOPE. 493 



iDack combination were examined by removing the ocular and 

 looking down the tube, it would be found, if he is using the edge 

 of the flame, that it is only partially filled with light, and that 

 wdth a streak image of the flame, if critical illumination is being 

 employed. 



It will be found that, with the Nelsen collecting lens, low-power 

 objectives require the lamp to be quite close (4 to 6 inches from 

 the mirror) ; medium powers (1/2 inch or l/4th inch) from 6 to 12 

 inches ; and high powers (l/6th to 1/1 2th) from 12 to 24 inches. 

 With the continental form of condenser these distances would 

 have to be increased. 



6. Critical and Non-critical Illumination. 



By " critical illumination " is meant that with the tube length 

 properly adjusted, and cover-glass correction made, the objective, 

 a,nd the source of light by means of the condenser, are both 

 focused on the same plane of the object. This arrangement has 

 been recommended by the best authorities, as giving the very 

 best possible results. It is, however, not uncommon to hear and 

 see statements, by quite experienced microscopists, that " critical 

 illumination " does not suit them, and that they can see nothing 

 with it. This therefore deserves some consideration. 



When we obtain, according to the above directions, so-called 

 critical illumination, and the back combination is completely filled 

 w^ith light, it will be found that very few lenses and that only 

 with certain objects will give perfect definition under such 

 circumstances until the substage iris has been somewhat closed. 

 When this partial closing of the iris has been carried out, just to 

 the extent of giving perfect definition, we have got, as far as I 

 understand the term, " critical illumination." 



As I have said, there are some who say that they .can see best 

 with non-critical illumination, and I thought it would be interesting 

 to carry out a few experiments, visually and photographically, to see 

 how one compared with the other; with the result that when 

 using a collecting-lens system of illumination, and jDrovided that 

 the same working ajjerture is used, critical and non-critical 

 illumination apparently gave identical results. With non-critical 

 it is easier to obtain uniform illumination of the field, especially 

 with the low powers, whilst with critical it is imperative that 

 the glass of lamp and collecting lens should be absolutely clean 



