1 6 PRACTICAL HISTOLOGY. 



the high power ; use a medium eye-piece, and focus a microscopic 

 preparation ; any thin section of a tissue will do. The field should 

 be large, well illuminated, and flat. If there are specks in it, clean 

 all the lenses to see that these specks are not due to dust on the 

 lenses. If the centre of a flat object in the field does not come into 

 focus at the same time as the periphery of the object, then the lens 

 should be rejected on account of its spherical aberration. If coloured 

 rings appear in an object, then the lens must also be rejected, as it 

 is not perfectly achromatic. The definition ought to be sharp and 

 distinct. 



The qualities of a good lens are the following : 



The definition should be good, i.e., the correction for spherical 

 and chromatic aberration should be perfect. The outlines of objects 

 should be sharply defined and not blurred, and there should be no 

 coloured halos or fringes round the object. If so, the chromatic 

 aberration of the lens is not perfectly corrected. 



Flatness of field, i.e., all parts of the object in the field at the same 

 time should be seen with equal distinctness, and, of course, this can 

 only be so when the field is flat. If the central parts are sharply 

 focussed, the peripheral parts not, then the field is not flat. 



Resolving power, which depends on the angle of aperture. This 

 is the power to render visible surface markings, superficial lines, or 

 structural details. -It is better to have a lens of large angle of 

 aperture for this purpose. 



Penetrating power, or the power to see objects in several planes 

 in the same preparation at the same time. It represents the focal 

 depth of the lens and its power of focussing images from different 

 planes in the vertical range. Narrow angled lenses are more suit- 

 able for this purpose. 



Working Distance, i.e., the distance between the lens and the 

 object. It has no direct relation to " the focal length of a lens. 

 Wide angle lenses have a shorter working distance than narrow angle 

 lenses. A good lens should combine all the foregoing qualities, but 

 of these qualities definition is all important. 



15. Drawing of Microscopic Objects. In this laboratory every 

 student is required to make sketches of his preparations. This is 

 of the utmost importance, not only from the point of view of the 

 student, but also of the teacher. 



(A.) Freehand Sketching. The student must provide himself 

 with a drawing-booli, (p. 3) and with suitable pencils (H.B. and 

 H.H.H.), and also with either coloured crayous or water-colours. 

 It is far more important that an outline sketch in pencil be made 

 accurately portraying the shape of the object, than that an indifferent 

 sketch should be covered with a smear of colour. It is astonishing 

 how some students, after protesting that they cannot " draw," 



