100 HOW TO SEE WITH THE MICROSCOPE. 



taxed in the daily routine of labor and study. Their 

 glasses were the best that could be procured when pur- 

 chased, and, as far as their personal experience with 

 them could attest, their experience was in harmony 

 with the laws laid down by Dr. Carpenter ; and they 

 had " no time to waste " in fussing over " diatom tests," 

 etc. 



On the other hand, the empiric replies, " Your high- 

 angled glasses are all well enough for the diatom man, 

 but for my work give me reliable French triplets of 

 moderate apertures;" and in this statement the author 

 is in entire and perfect harmony ! 



The author repeats, that ten years ago the doctrines 

 taught, as contained in the previous quotation of Dr. 

 Carpenter, were essentially true; he has no fault to 

 find with their original publication, but he does regret 

 that Dr. Carpenter has allowed them to retain their 

 place, unchanged or unrevised, in his late editions. 



Let us glance for a moment at the history of the ob- 

 ject-glass as connected with its aperture. Years since 

 it was known that the exhibition of surface-markings 

 required to a greater or leeser extent the employment 

 of lateral or oblique light; width of aperture hence 

 became a desideratum, at least in such glasses as were 

 to be used with this object in view. . 



Whether right or wrong, demands were made on the 

 optician for objectives possessing increased apertures. 

 Notably, this demand was first met successfully by our 

 own countryman , the veteran Charles A. Spencer, who 

 produced a glass of wider angle than had been pre- 



