A&P 1 lab help

Here's something I used. Hope it helps!

http://virtualurchin.stanford.edu/microscope.htm

The way it was explained to us is, you start at 4x and use the large knob on the side to bring it into focus. You then change the objective to 10x and from that point on, do not use the large knob...only the smaller one in the middle of it. You can then move on to 40x and 100x doing the same thing, only adjusting that inner knob and of course the side to side/front to back if need be. But you only use the largest knob when it's on 4x just to bring it into focus initially. Hope this helps!

I think you are talking about the focal length? If so, the focal length is the distance b/w the slide and the objective lens. For example, when viewing at 40X (using an objective lens of 4x and an ocular magnification of 10x) your focal length is 46mm. But when you increase the magnification to 1000x (using an objective lens of 100x and an ocular magnification of 10x) it is only 1.8mm. Note that the 100x objective is much longer than the scanning objective of 4x.

thanks for all the advice.our instructor is going to give us a table with the objective lenses we have to name them,and then give the total magnification on them and then estimate the field distance on each specimen that we are working on.kinda like looking at the specimen and finding out how big the specimen is.she will give us 3 out of the 4 numbers and we have to come up with the 4th number by doing a algebra equation with it.but she is no math teacher and didnt explain it well enough.