ZOOIiOaT AND BOTANY, MICKOSCOPY, ETC. 449 



wide aperture cannot be utilized, except by a method of illumination 

 that causes light to pass through the object at an obliquity corre- 

 sponding to that at which the most divergent rays enter the objective. 

 Now, although in the case of objects whose markings are only super- 

 ficial such may not be productive of false appearances (though even 

 this is scarcely conceivable), it must have that effect when the object 

 is thick enough to have an internal structure ; and the experience of 

 all biological observers who have carried out the most delicate and 

 difficult investigations is in accord, not only as to the advantage of 

 direct illumination, but as to the deceptiveness of the appearances 

 given by oblique, and the consequent danger of error in any inferences 

 drawn from the latter. Thus, for example, the admirable researches 

 of Strasburger, Fleming, Klein, and others upon the changes which 

 take place in cell-nuclei during their subdivision can only be fol- 

 lowed and verified (as the writer can personally testify) by exami- 

 nation of these objects under axial illumination, with objectives of an 

 angle so moderate as to possess focal depth enough to follow the 

 wonderful differentiation of component parts brought out by staining 

 processes through their whole thickness. 



The most perfect objectives for the ordinary purposes of scien- 

 tific research, therefore, will be obviously those which combine exact 

 definition and flatness of field with the widest aperture that can be 

 given without an inconvenient reduction of working distance and loss 

 of the degree of focal depth suitable to the work on which they are 

 respectively to be employed. These last attributes are especially 

 needed in the study of living and moving objects ; and in the case of 

 these, dry objectives are decidedly preferable to immersion, since the 

 shifting of the slide which is requisite to enable the movement of 

 the object to be followed is very apt to produce disarrangement of the 

 interposed drop. And, owing to the solvent power which the 

 essential oils employed for homogeneous immersion have for the 

 ordinary cements and varnishes, such care is necessary in the use 

 of objectives constructed to work with them, as can only be given 

 when the observer desires to make a very minute and critical exami- 

 nation of a securely mounted object." 



A table is then given which in addition to the magnifying-powers 

 of objectives with the A and B eye-pieces also " specifies the angle 

 of aperture which, in the writer's judgment, is most suitable for each. 

 He has the satisfaction of finding that his opinions on this latter 

 point, which are based on long experience in the microscopic study 

 of a wider range of animal and vegetable objects than has fallen 

 within the purview of most of his contemporaries, are in accordance 

 with the conclusions drawn by Professor Abbe from his profound 

 investigations into the theory of microscopic vision, which have been 

 carried into practical accomplishment in the excellent productions of 

 Mr, Zeiss." An extract from the table will be found on the next 

 page. 



" For ordinary biological work, the 1/8, 1/10, and 1/12 objectives, 

 with angles of from 100° to 200°, will be found to answer extremely 

 well if constructed on the water-immersion system." 



Ser. 2.— Vol. IV. 2 H 



