46 Modern Microscopy. 



essential that one of the drawtubes should be actuated by 

 rack and pinion, and it cannot be too strongly urged on 

 purchasers of instruments to try and obtain those having 

 this convenience. There is no doubt that as soon as the 

 demand is made the manufacturers will meet it with a 

 cheaper and equally efficient rackwork drawtube than the 

 expensive ones at present obtainable. 



It is a somewhat difficult matter for the novice to choose his 

 own object-glasses, as it is only by comparison he can judge 

 as to their merits. For objectives varying in power from 2 

 inches to J inch nothing is better as a test than the proboscis 

 of a blow-fly. The spines should be focussed and each show 

 a well-defined point ; a higher power eyepiece should then be 

 inserted, and any diminution in definition should be rioted. 

 We would here remark that it is always advisable to buy 

 these low-power objectives composed of double combina- 

 tions. Some of the cheaper ones are only composed of two 

 or three lenses balsamed together ; with these sufficient 

 aperture cannot be obtained, nor good definition combined 

 with flatness of field. All the best low-power lenses are con- 

 structed with two pairs of lenses a little distance apart, and 

 can be recognised immediately. Flatness of field is an im- 

 portant point in a low-power objective, and the best method 

 of testing for this quality is by either a stage micrometer 

 focussing the ruled lines, or a piece of ground glass. With 

 the higher powers this feature is sadly neglected, especially 

 in lenses of Continental make, it being advocated by one 

 or two leading workers that it is better to get the utmost 

 perfection of definition in one central point rather than 

 that definition should be in any degree sacrificed for 

 flatness of field. It would certainly be a great advantage 

 to the microscopist if the two points could be combined in 

 a more satisfactory manner than they are at present. The 

 English manufacturers, in their high powers, generally 

 provide a flatter field than their Continental contem- 

 poraries. As tests for objectives from J inch upwards, the 

 internal markings of Triceratum, the striae of Pleurosigma 



