OF THE MICROSCOPE. 63 



still the question seems to bs undecided. Carpenter, in his last 

 edition (page 702) says: "The sharp and distinct bringing-out 

 of the ' exclamation marks ' of the Podura scale, constitutes, when 

 it coexists with the greatest practicable freedom from color, and 

 with adequate * focal depth ' or 'penetrating power,' the most 

 valuable proof of the fitness of an Objective of high power for 

 the purpose of scientific work." 



To give our readers an idea of 



how the podura scale ought to look, 

 we give a figure copied from the 

 engraving published by the late 

 Richard Beck, in his work on the 

 microscope. The figure shows 

 " the appearance of the Podura 

 scale when the adjustment of the 

 object glass is correct and the mark- 

 ings are in focus." The objective 

 used was a one-eighth, giving a 

 Fig. 6. magnifying power of 1,300 diame- 



ters. 



It is, we believe, generally conceded that in the present state 

 of the art, perfect correction for color cannot be obtained, but 

 so long as the residuary chromatic abberration does not inter- 

 fere with the defining power of the objective, it cannot be re- 

 garded as an objection. And yet we have seen a would-be critic 

 reject a very excellent lens because it showed a little color, 

 while he was loud in his praise of another lens which, although 

 more perfect in this respect was almost worthless otherwise. 

 Like specks of dirt on an eye-piece, which do no harm and are 

 never even seen by experienced microscopists, slight color and 

 want of flatness of field are the betes noirs of beginners. They 

 are the defects which are most easily detected, and the detec- 

 tion gives the critic an air of knowledge which is to him a 

 source of great pride. 



The best English and American opticians now slightly under- 

 correct their best objectives, so that the field shows a slightly 

 greenish hue, while any prominent markings on the object, 

 such as the dots on the Angulatum stand out clear and well de- 

 fined and of a very delicate ruby tint. According to Frey the 



