MEDIUM-POWER PHOTO-MICROGRAPHY 87 



is its depth of focus. It would seem now still more than ever apparent that the day 

 for low-angled objectives is gone, but we again repeat this is not quite the case, for 

 when it is desirous to take in at a glance the entirety of the dimensions of an object, 

 its height, depth, and width, and the place it occupies amidst others that surround it, 

 we may find it far more enlightening to use a low-angled objective with plenty of 

 depth of focus. It is better, we may find, to sacrifice the ideal in definition so as to 

 obtain a more realistic picture — one of greater utility and instruction, seeing it would 

 enable the eye of the observer at a glance to perceive the perspective and realise the 

 general relations of the part to the whole than would be obtained in the case of 

 the mental image only derived by differentially focussing first one plane and then 

 another, as must be done with a high-angled objective. 



This is true so far as vision is concerned, but when wishing to photograph what 

 we see, the effect of a low N. A. and depth of focus may be even more apparent, for 

 when visibly we inspect an object in the microscope another factor enters into the 

 problem, to which at present we have only referred some pages ago (page 85) — the 

 accommodation of the eye itself According to Professor Abbe, it assists very largely 

 the performance of all objectives so far as relates to their depth of focus ; but when 

 taking a photograph there is no eye to assist, hence what appears well in focus to the 

 observer, may not be so in the negative, for no amount of dexterity will give to the 

 plate that part of the depth focus which has been due to the eye. Consequently 

 objectives of low aperture may be even of more use to the photo-micrographer than 

 to the visual observer. 



Each type, then, of lens has its duty to fulfil. But if the reader is limited to the 

 purchase of one kind only, we again state our conviction that the high-angled lens is 

 that to be selected. 



Shutting the iris diaphragm below the condenser is known to practically reduce the 

 N. A. of the objective being employed, hence the question has been asked, If this is 

 true, why buy low- angled lenses at all ? It is because the image produced by closing 

 the iris is not equal in truth or efficiency, owing to diffraction phenomena, to that 

 given by a low-angled objective made as such. 



Owing to apochromatics performing so well with any eye-piecing, anyhow, up to 27, 

 or even more under certain conditions, this type of lens is not made of very low angle, 

 for one which does not possess a great N. A. can be eye-pieced to give the required 

 magnification, and so the depth of focus so far as results from the low N. A. can thus 

 be obtained. 



Seeing that the numerical aperture is of such importance, it must now be pointed 



