real*4 l,lambda,d,dprime
	write(6,*)' assuming lambda=1.54178, true spacing 3.683 A'
	lambda=1.54178
	d=3.683
	write(6,*)'enter assumed length and observed ',
     .  'd-spacing of ice 0 0 2 reflection:'
	read(5,*) lprime,dprime

	x=tan(2*asin(lambda/(2*dprime)))
	l=lprime*x/(tan(2*asin(lambda/(2*d))))
	write(6,*) 'more accurate camera length:',l
	end