428 The Eye and the Microscope. 



neighbours, which has never been employed in seeking the 

 information which lenses are able to afford. 



In the earlier days of optical science, it was impossible to 

 imitate the natural conditions of good and pleasant sight, 

 either with the microscope or the telescope. The eye possessed 

 advantages which no artist was then able to reproduce in 

 optical combinations ; and hence every attempt to extend 

 its powers involved the necessity of putting up with serious 

 defects. These unfavourable circumstances have been so 

 far changed, that it is now possible to make telescopic, or 

 microscopic, vision almost as clear and as easy as if no instru- 

 ment intervened. To do so, however, requires a close copy of 

 the natural conditions of satisfactory sight. We see objects 

 most agreeably and correctly at a certain distance, with a 

 certain quantity and a particular direction of light ; and it is 

 also necessary that they should be, or appear to be, of a suffi- 

 cient size. To stare at a white house in the full glare of sun- 

 light, to strain every nerve in order to make out dim outlines 

 faintly and uncertainly looming through a fog — these are 

 things which everybody knows are painful and mischievous ; 

 and if we make our microscopic experiments under a similar 

 blaze, or in a similar mist, we shall easily produce an analo- 

 gous effect. If, on the other hand, the light falls softly, and in 

 right quantity, upon or through our object — if the power em- 

 ployed is sufficient to make the details clear, and the whole 

 arrangement is good — the sensation will bear so close a resem- 

 blance to that of the normal vision, that little or no fatigue will 

 be felt. 



This question is important, because on the one hand many 

 persons hesitate to become microscopists from a fear that their 

 eyes will not bear the strain ; and, on the other hand, scores of 

 people who have purchased instruments, give up their use 

 because they do not succeed in seeing through them with 

 satisfaction and ease. The first essential requisite is to obtain 

 a microscope free from important defects, and unless it is con- 

 venient to spend a large sum of money at once, it is better to 

 have only two powers, and a sufficient quantity of apparatus to 

 use these with the best effect. Furnished with a microscope 

 having a body about ten inches long, or if shorter with a 

 Kelner eye-piece — which to some extent remedies the want of 

 length — and two objectives of one inch, or two-thirds, and one- 

 quarter, or one-fifth focal lengths, and with two or three eye- 

 pieces, the student can verify the majority of observations of 

 general interest in the animal and vegetable- kingdoms, and he 

 can examine for himself all the objects he is likely to collect. A 

 greater array of powers is often convenient, and even indispen- 

 sable for special purposes ; but it is of far more consequence 



