622 SUMMARY OF CUBRENT RESEARCHES RELATING TO 



A really critical image could only be seen with artificial light, and 

 with a good condenser and diaphragms. He does not mean to say 

 that no good work can be done with diffused daylight, for excellent 

 work is done with low or medium powers ; but he insists that it is not 

 possible to do any such critical work as testing objectives by daylight 

 as thoroughly as it can be done by artificial light. With daylight and 

 mirror only there is milkiness and " glaze." The milkiness can be 

 got rid of by a diaphragm, and the " glaze " by using a ground glass 

 behind the object. Unless a condenser is used there will always be 

 found a falling off in the quality of the image with all powers higher 

 than 2/3 in. From long experience in working with the Microscope, 

 he feels justified in asserting that on the whole daylight is more trying 

 to the sight than lamplight. 



The oxy-hydrogen light may be serviceable for resolving such 

 tests as Nobert's lines, but the incandescence lamp he regards as 

 entirely a failure for microscopical purposes. " This is at once 

 obvious upon the consideration that the finest images seen are got by 

 viewing objects, as it were, in the image of the source of light. All 

 critical images of transparent objects viewed by direct transmitted 

 light require first that the source of light should be pictured by the 

 condenser exactly in the plane of the object, the object then serves 

 to interrupt the image of the source of light. The observer has simply 

 to arrange the lamp, condenser, and diaphragms so as to produce the 

 most perfect image of the source of light of the required size in the 

 plane of the object, the objective will then have fair play. The size 

 of the image of the lamp flame can be controlled by distancing the 

 lamp. There is no other secret in the matter. With the incandescent 

 lamp the image produced by the condenser represents the mere 

 carbon thread, on which no object could be seen projected ; in order 

 to obtain some extent of brightly luminous field, the condenser must 

 be put out of focus, then the intensity of the light is so reduced that 

 the observer would simply discard the incandescence, finding it far 

 less serviceable than a shilling paraf&n lamp." 



He entirely condemns the use of paraboloids for dark-ground 

 illumination, as properly adjusted central stops with the condenser 

 will give by far the best dark-ground illumination. For opaque 

 objects he considers nothing has been devised so good as Lieberkiihns, 

 and objects ought as far as practicable to be mounted for use with 

 Lieberkiihns, and not covered up with paper. If the side illuminator 

 is used it should be attached to a fixed part of the stand, not to the 

 body-tube or stage. 



With the preceding remarks may be contrasted the view of 

 Prof. Abbe (in litt.) that it is quite immaterial, from a theoretical 

 point of view, whether an illuminator has or has not spherical 

 aberration. The effect of illumination does not depend upon the 

 projection of a sharp image of the source of light upon the object, nor 

 even on the projection of any image at all. The only object of pro- 

 jecting an image of the source of light approximately at the plane 

 of the object is in order that a uniform illumination of a given area 



