4 LIVING CELLS (1) 



Exercise I 



anatomy of cells to that of subcellular particles. 

 It has also made the larger molecules visible, 

 for 10 angstrom units corresponds to the diam- 

 eter of a rather small protein. The larger pro- 

 teins and the nucleic acids and viruses can readily 

 be distinguished under the electron microscope, 

 and at times even identified by their characteris- 

 tic shapes. 



To go further requires radiations of still 

 shorter wavelengths (x-rays). Methods of x-ray 

 diffraction, simple in principle though compli- 

 cated and laborious in practice, permit us to 

 determine the positions of the individual atoms 

 in molecules. Here the limit of resolution is a 

 fraction of an angstrom unit. With such meth- 

 ods the characteristic distances and angles be- 

 tween the atoms in molecules can be determined. 



This is the last reach of anatomy. It is not 

 only that biological interest does not penetrate 

 further; that might change with time. It is 

 rather that beyond this point, anatomy becomes 

 indeterminable. Particles smaller than atoms are 

 subject to the limitations of physical indeter- 

 minacy; it is impossible to assign definite 

 meaning to their individual spatial relations. 



So the domain of biological anatomy stretches 

 over an enormous range, from gross anatomy 

 to molecules, from centimeters to angstrom 

 units, from the anatomy of the dissecting pan 

 to that of x-ray crystallography. There are no 

 sharp boundaries; and nowadays biologists 

 must be concerned with the entire continuum. 



Use of the microscope 



A compound microscope is a delicate and 

 expensive instrument. Treat it tenderly. Don't 

 begin by twiddling knobs, and then trying to 

 find out what you twiddled. Find out what to 

 do before you do it. Your instructor, charts, 

 booklets, this outline — all will help. 



The following summary of practical directions 

 will get you started : 



(1) Pick the microscope up by its arm. Don't 

 let it knock against anything, and set it down 

 gently to avoid jarring its parts out of line. 



(2) Place the microscope on your desk with 

 the arm toward you. Identify the ocular (eye- 

 piece), nosepiece and objectives, the body tube, 

 the coarse and fine adjustments for focusing, 

 the stage, iris diaphragm, condenser (if present), 

 and the mirror — one face of which is plane, 

 the other concave — which reflects light through 

 the opening in the stage into the objective. 



(3) Revolve the nosepiece to bring the low- 

 power (the shorter) objective into line with the 

 body tube. A spring catch engages the nose- 

 piece to hold each objective in its correct posi- 

 tion. 



(4) Adjust the mirror so that the concave side 

 reflects light from your microscope lamp through 

 the opening in the stage into the objective. If 

 the light is too intense, close down the iris 

 diaphragm until the brightness suits you. 



(5) Always look at the object first under low 

 power. Place a slide on the stage with its ends 

 held by the spring clips (or in a mechanical 

 stage), and position it so that the part you want 

 to study lies at the center of the opening. Lower 

 the tube until the bottom lens of the objective 

 is about a quarter-inch above the slide. Now, 

 while looking through the ocular, focus slowly 

 upward with the coarse adjustment, until the 

 image is visible. Continue to focus up and down 

 with the fine adjustment until the image is 

 sharpest. 



(6) To use the high-power objective, first 

 focus the object under low power and move the 

 slide until the detail you wish to examine under 

 higher magnification is almost exactly centered 

 in the field. Turn the nosepiece slowly to swing 

 the high-power objective into line. Watch 

 meanwhile from the side, to see that this longer 

 objective does not strike the slide or cover glass. 

 Now focus slowly upward with the fine adjust- 

 ment (the coarse adjustment is never used with 

 the high-power objective). If no image is seen, 

 carefully lower the objective until it almost 

 touches the slide, then focus upward again. 

 The point is to avoid any possible damage to 

 the objective by permitting it to touch the slide. 

 Finally, bring the image into sharpest focus by 

 playing back and forth with the fine adjustment. 



