The Microscope. 



they have made their glasses perfectly homogeneous, 

 that is, they require no change of the oil when using 

 oblique light on an object, and no correction with the 

 draw tube is necessary when using a very thin cover. 

 With Zeiss' -^ it is necessary to draw out the tube two 

 inches with a '004 cover glass, and for central light a 

 mixture of fennel and olive oils is required, which must 

 be changed to cedar oil for oblique light. 



With Messrs. Powell and Lealand's glasses cedar oil 

 is used indifferently for central or oblique light, and no 

 correction is necessary for a thick or thin cover glass. 



In using oil immersions, all that is required is to 

 place a very small drop of oil on the front lens, screw 

 the glass into the body, and lower it on to the slide until 

 contact is made, which can be seen by bringing the eye 

 to a level with the surface of the slide. The glass is 

 then focussed by the fine adjustment until the object is 

 seen sharply defined. With these glasses the best effect 

 is obtained the moment the glass is in focus, and they 

 have an enormous superiority over a dry glass of the 

 same power in sharpness of definition and brilliancy. 

 All slides intended for use with high powers should be 

 sealed with Hollis' glue, and then the cedar oil left on 

 the cover glass can be wiped off without the least dan- 

 ger to the preparation. 



Eye- Pieces. Students' microscopes are generally sold 

 with two eye-pieces, nos. 1 and 3, or A and C, but the 

 no. 1 or A is the only one required, as no good result is 

 obtained by using an eye -piece with a higher magnify- 

 ing power, the higher eye-piece only magnifying the 

 image seen with the object glass. 



