200 A A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 



especially in photomicrography, it should be capable of yielding 

 a solid cone of N.A. 5. Now, it is a remarkable fact that 

 no cheap very-low-power achromatic condenser of the kind 

 above indicated is at present on the market, so that clearly 

 no great demand for it exists, although there must be hundreds 

 of earnest investigators to whom it would prove a boon could 

 they once realise the advantages derivable from it. As a 

 makeshift it will be found, however, that one of the ordinary 

 triplet combination loups magnifying 6 times, when mounted 

 in a suitable fitting and provided with an iris diaphragm, will 

 answer well for 2-in. to l|-in. objectives, affording a large 

 image of the light source, and practically filling the field when 

 the broad part of the lamp flame is focussed. The Zeiss Abbe 

 ochromatic condenser with top lens removed gives even better 

 results ; and I would especially call attention to the modified 

 semi-apochromatic form recently introduced by Baker, which, 

 without the front lens, has a power of y^j-in., and an ajjlanatic 

 N.A. of '5, thus rendeiing it available for use with a wide- 

 angled I -in. apochromat, while with the top lens in situ, it 

 may be satisfactorily employed with the higher powers, being 

 then a y^-in. of aplanatic N.A, "9 ; but in this latter respect 

 the high power dry and oil-immersion condensers of all our 

 leading opticians now leave little to be desired. 



Regarding the third point — i.e., the apparent carelessness and 

 lack of interest in all optical matters evnnced by many 

 naturalists — it is difficult to believe that such men, most of 

 whom will put themselves to infinite pains in the collection, 

 careful preparation, and preservation of their specimens, could 

 fail to be equally fastidious and painstaking with respect to 

 the manner in which thev exhibited to themselves and others 

 the fruit of their hard work, toil none the less taxing because 

 it happens to be entirely a labour of love, without thought of 

 gain, were they once convinced that any practical advantage 

 might be derivable in their own particular branches of research 

 by a little more careful attention to optical principles. In 

 some instances these workers may be deterred from any attempt 

 to grapple with the subject by the mistaken idea that con- 

 siderable study of an intricate nature in a, to them, uncongenial 

 field is requisite, or that the cost of a proper instrumental 

 outfit to secure optical results of a high order is quite beyond 



