SOMETHING FURTHER ABOUT OBJECTIVES. 119 



as generally taken and accepted may be thus stated: 

 objectives of low angular aperture are endowed with a 

 peculiar inherent and intrinsic power, by virtue of 

 which they enable the observer to see and study struc- 

 tures situated in different planes of the object. For 

 example, if the objective be focussed accurately to 

 details occupying an intermediate plane of the object to 

 be examined, then will the low-angled glass allow the 

 observer, without change of focus, to study other 

 details of the said object, situated in planes either 

 nearer or more remote. We have been taught that this 

 is a most valuable property, and one due to the employ- 

 ment ot low angles only the idea thus conveyed being 

 that the low angles possess a peculiar and accommodat- 

 ing power of great value to the microscopist, to which 

 the wide apertures stand inflexibly opposed, and defiant. 

 In support of the doctrine of penetration , it has been 

 customary to present the case of the optical principles 

 governing the action of low apertures, contrasting the 

 same relatively with similar conditions involved in the 

 use of the high angles ; thus we have been taught that 

 the narrow-angled glass admits as a matter of course, 

 but a narrow cone of light, the pencils crossing at the 

 focal point at a very acute angle. Hence it is "obvious" 

 that it matters not whether the object to be viewed be 

 placed exactly at the crossing point or a little within or 

 without the said focus. The accompanying and sup- 

 posed increase of working distance attributable to the 

 narrow aperture of course is not lost sight of; and we 

 are here admonished to keep in mind the fact that, with 



