Instructions for Veho VMS--400 USB microscope
I write this as a professional microbiologist. This microscope is excellent but, at first, it seems impossible to use at high power [400x] because of inadequate instructions. I recommend practising on a simple flat object like a small piece of printed paper. Press the microscope firmly down onto it [or put the object into the lens cap] and focus at 20x. Now use the focus wheel to change to high power by rotating anticlockwise as far as it will go from that 20x position and then very slowly moving back until focus is achieved. [Note that it looks as if it should be possible to rotate a short distance from the 20 mark to the 400 mark on the focussing wheel. This is not the case. It is necessary to turn the wheel in the opposite direction to the 400 mark.]
As others have commented it is difficult to focus at 400x because the depth of field is small, the contrast is low and it is difficult to hold everything steady. This is not a fault of this microscope; it is true for the best professional instruments which overcome the problem by being very heavy, having precision engineered platforms for moving the object and special lighting systems (e.g. phase contrast). With the VMS-400 I recommend practicing focussing on a simple object as described above; it also may help to fix it to a heavy base or to place the object in the transparent lens cap. For smaller objects it is helpful to press them between glass slides for viewing at high power.