OPTICAL IMPROVEMENTS. 71 



but so obliquely, that none of it enters the object-glass 

 but that which is interrupted by the object. 



5. Mr. Brooke's double nose-piece, or Baker's treble, is 

 a useful piece of mechanism attached to the end of the 

 microscope body, by which two or three object-glasses 

 can be screwed on to it at once, and rapidly changed. 



6. The double arm to the mirror allows it to be ex- 

 tended so as to cast a very oblique light on objects, as 

 well as to be raised near them without any preparatory 

 movement. 



7. The separation of the outer and inner lenses of object- 

 glasses is an arrangement by which the greatest possible 

 flatness of field and penetration are secured. 



8. The great and manifold advantages of the binocular 

 arrangement are very obvious ; objects stand out with 

 stereoscopic effect, and the ease to the eyes of the observer 

 is very great. When high powers are required, the prism 

 can be drawn aside, and the microscope used as a mon- 

 ocular instrument. 



As regards recent optical improvements, Mr. Ross (the 

 elder), as we have already explained (page 40), was the 

 first to notice that however perfect the correction of an 

 object-glass might be, " the circumstance of covering the 

 object with a piece of the thinnest glass or talc, disturbed 

 the corrections if they had been adapted to an uncovered 

 object, and rendered an object-glass which was perfect 

 under one condition, seriously defective under the other." 

 To remedy this defect, he devised the well-known contri- 

 vance called the adjustment of object-glasses, by which 

 they are rendered equally correct for covered or uncovered 

 objects 



The following diagrams (fig. 38) of the different object- 

 glasses will help to explain the complex structure, and 

 consequent costliness, of an achromatic combination. The 

 double-convex and plano-convex lenses are of crown glass, 

 and the piano- and double-concave, and meniscus lenses, 

 of flint glass. 



It will be seen, therefore, that each of the object-glasses 

 from the J to the ^V, is made up of as many as eight dis- 

 tinct lenses, the back combination being a triplet composed 

 of two double-convex lenses o'f crown, with a double- 



