CH. //] 



LIGHTING AND FOCUSING 



45 



ually push the draw-tube into the tube. If this were done without 

 these precautions the objective might be forced against the object 

 and the ocular thrown out by the compressed air. 



§ 84. Focusing with High Objectives.— Employ the same 

 object as before, elevate the tube of the microscope and, if no revolv- 

 ing nose-piece is present, remove the 16 mm. (fi in.) objective as 

 indicated. Put a 4 or 3 mm. (\ or \ in.) or a higher objective in 

 place, and use a low ocular. 



^22 2" Ejr.-lw 



Virtual (.TOO^ ^ 



Hit Jr»|.ura$ru 





ruu i»o» 



Fig. 46 



"This arrangement and no other one fulfills at the same time the other 

 request that the amplification of the microscope with different oculars should 

 be in exact inverse proportion of the equivalent focal length of the oculars." 

 " The position of the point D* may be easily calculated for every ocular. 

 If A is the distance of the diaphragm from the field lens and X the focal length 

 of that lens, the distance of the focus D* above the diaphragm (z. e. the dis- 



A= 



tance from D to D*) will be: fd= . Hoping that these explanations 



X— A 

 will be found satisfactory for your aim, I remain yours sincerely, 



DR. E. ABBE." 

 Oa p. 31 of the Bulletin is the following : " Par- focal Eye-pieces. Referring 

 to the article in the April issue of the Bulletin, on changing eye-pieces with- 

 out altering focus, etc., we announce that we are prepared to furnish eye- 

 pieces as there described with our Acme microscopes at a slight additional 

 expense. 



We have named these eye-pieces Par-Focai,, meaning of equal focus, 

 from the Latin par (equal) and focus.'' 1 



