316 HOW TO SEE WITH THE MICROSCOPE. 



The author learns from good authority that Mr. 

 Zeiss has recently extended the balsam aperture of his 

 oil-immersions improving thereby the performance. 



Oil-immersions are now made by Mr. Tolles and Mr. 

 Spencer. These makers, however, supply the collar 

 adjustment (as should have been done by Mr. Zeiss). 

 Mr. Spencer's objectives we are informed are arranged 

 so as to work with oil or glycerine contact. 



While in the oil business as above stated, it occurred 

 to me to try the effect of various oils with my duplex 

 glasses. After a little experimenting, I found that 

 ordinary kerosene, well washed in alcohol, worked 

 most beautifully with both the Tolles one-tenth and 

 the Spencer duplex one-fourth, with either glass when 

 thus immersed I got the most exquisite displays of the 

 most difficult known tests. It thus came to the surface 

 that I had harbored oil immersions while ignorant of 

 the fact. 



That the cedar oil diminishes the range required in 

 process of adjusting for varying thickness of cover 

 must be granted, hence the novice using the Zeiss im- 

 mersions being necessarily restricted to this more lim- 

 ited range might possibly get better initial perform- 

 ance than would result in his first attempts to manipu- 

 late a Tolles or Spencer four-system objective. 



In dispensing with the cover adjustment and its 

 accompanying mechanism (such as furnished by Spencer 

 or Tolles), falling back on the clumsy draw-tube as a 

 substitute, the cost of construction is materially re- 

 duced. Those affected with the res angusta dorni might 



