1883.] 



MICROSCOPICAL JOURNAL. 



12- 



with geometrical prisms of carbonate 

 of lime in planes one over the other, 

 and often radiating from different 

 points. Shadow and high liglit suc- 

 ceed when the Nicol is rotated, and 

 minute details become apparent which 

 are not seen, or are only feebly de- 

 fined, by ordinary light reduced in 

 its intensity to that of the polarizing 

 ray by the use of diaphragms. 



Some time since, in investigating 

 the structure of a fossil which was 

 composed of close radiating and oc- 

 casionally inosculating tubes with 

 very thin walls and a distinct lumen, 

 all mineralized with calcite in the 

 glassy, non-crystalline form com- 

 monly seen in fossils where there is 

 much space unoccupied by structure, 

 the polarizing ray certainly made the 

 tubes more distinct than the ray re- 

 flected from the mirror alone, and by 

 rotating the substage Nicol, the posi- 

 tion of certain tubes which were in- 

 visible before could be ascertained ; 

 that is to say, dark lines appeared 

 limiting tubes, which were invisible 

 under ordinary illumination. 



The application of the whole polar- 

 izing apparatus is very useful in work- 

 ing at the minute superficial struc- 

 tures made up of thin and highly 

 fractive plates of organic carbonate 

 of lime. The glare of light under ordi- 

 nary illumination, and even when the 

 polarizer only is used, prevents the 

 true surface being focussed, or if it is 

 fortunately hit upon, it is more or 

 less invisible. But the analyzer be- 

 ing placed across the direction of the 

 polarizing ray the true surface can be 

 found by the definition and distinct- 

 ness of the clear colors and the inter- 

 mediate lines. Take away the ana- 

 lyzer, and often new structures 

 appear to the eye. As a matter of 

 practice I find that this method is 

 exceedingly useful. 



Circumstances have brought me in 

 contact with cheap microscopes, and 

 certainly whilst it may be said that 

 some of the objectives are fairly 

 good, the eye-pieces are on the mis- 

 erable "par" with the rest of the 



apparatus. I cannot avoid believing 

 that during the next few years atten- 

 tion will be paid to increasing the 

 merits and adaptability of eye-pieces 

 whatever may be their special char- 

 acter. 



In the address which I had the 

 honor of delivering to you last year, I 

 remarked upon the comparative val- 

 ues of object-glasses with high and 

 low nnumerical apertures, and I took 

 pains to defend the employment of 

 lenses with wide apertures in exam- 

 ining minute objects, and also to 

 state that both kinds of objectives 

 are necessary for investigating into 

 the structure of minute objects. I 

 suggested what has commended it- 

 self to every advanced microscopist 

 for years past, that an observer 

 should provide himself with both 

 classes of objectives, and that he 

 should use those with a moderate ap- 

 erture for common and preparatory 

 work, and those with a high numeri- 

 cal aperture for subsequent and care- 

 ful examination. 



Very few microscopists care to 

 correct their objectives during ordi- 

 nary work, and. principally because 

 they have not seen the difference 

 made in the appearance of an object 

 by the process when it has been care- 

 fully carried out. But when an object, 

 hitherto unsatisfactorily defined, pre- 

 sents itself under a clear and definite 

 aspect, conversion to the opinion that 

 there is an absolute necessity for cor- 

 rection in all delicate investigations 

 regarding minute structures speedily 

 ensues. There is no doubt that with 

 very few exceptions the microscopic 

 work relating to the morphology of 

 the animal and vegetable kingdom has 

 been conducted either without cor- 

 rected objectives, or with those which 

 have an average adjustment. I point- 

 ed out in my last address how abnor- 

 mally thick, slender and excessively 

 minute bodies appear under a high 

 amplification ; this is partly due to a 

 want of correction, and mainly to 

 another cause which is not necessary 

 to advert to. Now, I have no hesita 



