ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



283 



character of the colour bands can be easily estimated. Differences in 

 the thickness of the cover-glass within the ordinary limits are scarcely 

 noticeable with such objectives. 



It is of fundamental importance in employing the test-plate to 

 have brilliant illumination and to use an eye-piece of high power. 

 With oblique illumination, the light must always be thrown perpen- 

 dicularly to the direction of the lines. 



When from practice the eye has learnt to recognize the finer dif- 

 ferences in the quality of the outlines of the images, this method of 

 investigation gives very trustworthy results. Differences in the thick- 

 ness of cover-glasses of 0"01 or 0'02 mm. can be recognized with 

 objectives of 2 or 3 mm. focus. 



The quality of the image outside the axis is not dependent on 

 spherical and chromatic correction in the strict sense of the term. 

 Indistinctness of the outlines towards the borders of the field of view 

 arises, as a rule, from unequal magnification of the different zones of 

 the olDJective : colour bands in the peripheral portion (with good 

 colour-correction in the middle) are always caused by unequal magni- 

 fication of the different coloured images. Imperfections of this 

 kind, improperly called ' curvature of the field,' are shown to a 

 greater or less extent in the best objectives, when their aperture is 

 considerable." 



How and Co.'s Pocket Lamp. — The feature of this lamp (fig. 50) 

 is its portability, having been 



constructed for microscopists ^^^- ^^^ 



who are in the habit of ex- 

 hibiting at soirees, &c. It fits 

 into a cylindrical tin case which 

 is small enough to be carried 

 without inconvenience in the 

 coat-pocket. When charged 

 with sufficient oil to burn for 

 about 3^ hours it weighs less 

 than 12 oz. As the foot is small, 

 the pillar upon which the lamp 

 slides has been made square, so 

 that the centre of gravity is 

 less likely to be disturbed. 

 The lamp can be elevated so 

 that the flame is 8 inches above 

 the table, but if greater height 

 is required, an additional two 



inches may be gained by standing the lamp upon the lid of the tin 

 case and fixing it by means of a screw provided for the purpose. 



Drawings and Paintings from the Microscope.* — The best 

 series of coloured drawings of microscopical objects that have been 

 seen within the memory of the present generation of microscopists, 

 were those drawn and exhibited by Mr. E. T. Draper at the April 



* Science-GosBip, 1882, pp. 1-3, 74, 203. ] 



