The use of shadow-casting technique for measurement of the width of elongated particles.

A method is described for the estimation of the true width of fibrillar or rod-like structures from electron micrographs of metal-shadowed preparations. The method is based on variations in the image width as a function of the angle (beta) between the long axis of the fibril and the direction of the shadow in the plane of the preparation. The image width when beta = 0 degrees practically represents the real width of the elongated particle but is often indistinguishable from the background. The fibril image width is conveniently measured at beta values between 15 degrees and 90 degrees . The true width is obtained by plotting the image width versus sin beta and extrapolating to beta = 0 degrees . Latex spheres are sprayed with the fibrils or rods to indicate the direction of shadow. Tobacco mosaic virus (TMV) was used as a model structure because of its known constant diameter of 150 A (5). The width (in the case of TMV equal to the diameter) found by the present method was 150 A +/- 8 A.